Google Apps Script Email System with Full UI - Allows for multiple people to be alerted on a form submissions. The program has a full user interface, menu and a sidebar that save the options a user selects.

Code.gs

// UI Sheets Email Notifications
// Kurt Kaiser
// kurtkaiser.us
// All Rights Reserved, 2019

// Declare global variables
var ss = SpreadsheetApp.getActiveSpreadsheet();
var sheet = ss.getActiveSheet();
var lastRow = sheet.getLastRow();
var lastColumn = sheet.getLastColumn();
var scriptProperties = PropertiesService.getScriptProperties();


// ------------------------ Create User Interface ------------------------

function onOpen() {
  var ui = SpreadsheetApp.getUi(); // Or DocumentApp or SlidesApp or FormApp.
  ui.createMenu('Email System')
    .addItem('Form', 'showFormSidebar')
    .addSeparator()
    .addSubMenu(ui.createMenu('Misc')
      .addItem('Delete', 'deleteData'))
    .addToUi();
}

function showFormSidebar() {
  var html = HtmlService.createHtmlOutputFromFile('Form')
    .setTitle('Edit Data')
    .setWidth(300);
  SpreadsheetApp.getUi()
    .showSidebar(html);
}

// ------------------------ Save Sidebar Options ------------------------
function userInput(form) {
  Logger.log('userinput');
  scriptProperties.deleteAllProperties();
  scriptProperties.setProperty('emailSubject', form.emailSubject);
  scriptProperties.setProperty('messageTitle', form.messageTitle);
  scriptProperties.setProperty('messageBody', form.messageBody);
  scriptProperties.setProperty('numToEmail', form.numToEmail);
  scriptProperties.setProperty('userEmailRadio', form.userEmailRadio);
  if (form.userEmailRadio == 'onUserEmailRadio')
    scriptProperties.setProperty('userEmailColumn', form.userEmailColumn);
  scriptProperties.setProperty('emailsString',  makeEmailsString(form));
  scriptProperties.setProperty('newEmailBoxes', addEmailBoxes(form));
  scriptProperties.setProperty('buttonRadio', form.buttonRadio);
  if (form.buttonRadio == 'onButton')
    scriptProperties.setProperty('buttonURL', ss.getUrl());
  // logProperties();
}

// --------------- Returns Array of All Properties  ---------------------
function getAllProperties() {
  var propertiesAndKeys = {}
  var data = scriptProperties.getProperties();
  for (var key in data) {
    propertiesAndKeys[key] = scriptProperties.getProperty(key);
    // Logger.log('Key: %s - %s', key, data[key]);
  }
  return propertiesAndKeys;
}

// Used to turn sidebar emails into a single string for sending
function makeEmailsString(form) {
  var emailString = '';
  var email;
  for (var i = 1; i < parseInt(form.numToEmail) + 1; i++) {
    email = form['email' + i];
    emailString = emailString + email + ',';
  }
  emailString = emailString.substring(0, emailString.length - 1);
  return emailString;
}

// Creates additional html email address boxes for the sidebar
function addEmailBoxes(form) {
  var amount = parseInt(form.numToEmail);
  var html = ''
  for (var i = 1; i < amount + 1; i++) {
    var value = form['email' + i] || '';
    html = html + i + ':<input type="text" value="' + value + '" id="email' + i +
      '" name="email' + i + '"/><br>';
  }
  return html;
}

// Limits the Saved message in the sidebar to 6 seconds
function waitSeconds() {
  Utilities.sleep(6000);
}

// Deletes all properties
function deleteData() {
  scriptProperties.deleteAllProperties();
}

// ----------------- Log Script Properties ------------------ 
function logProperties() {
  Logger.log('Log Properties');
  var scriptProperties = PropertiesService.getScriptProperties();
  var data = scriptProperties.getProperties();
  for (var key in data) {
    Logger.log('Key: %s, Value: %s', key, data[key]);
  }
}

// ----------------- Send Email on Form Submission ------------------
// Once a form, Google Form is submitted to spreadsheet
function onFormSubmission() {
  var properties = getAllProperties();
  var submission = getSubmissionString(properties);
  var allEmails = checkUserEmail(properties);
  // Makes an email template, updates it with saved data
  var htmlEmail = HtmlService.createTemplateFromFile('Email');
  htmlEmail.messageTitle = properties.messageTitle;
  htmlEmail.messageBody = properties.messageBody;
  htmlEmail = htmlEmail.evaluate().append(submission);
  htmlEmail = htmlEmail.getContent();
  emailSend(properties, htmlEmail, allEmails);
}

// Get sheet header row and last row submitted for html email output
function getSubmissionString(properties) {
  var submission = '<div class="box">';
  for (var i = 1; i < lastColumn + 1; i++) {
    submission = submission + '<span id="label">' + sheet.getRange(1, i).getValue() +
      '</span><br><span id="info">' + sheet.getRange(lastRow, i).getValue() + '</span><br>';
  }
  // Check is button is on in sidebar
  if (properties.buttonRadio == 'onButton') {
    submission = submission + '<br><a href="' + properties.buttonURL +
      '" id="button">View</a><br><br>';
  }
  submission = submission + '</div><br><p style="font-size:12px;'+
   ' text-align:center;">Created by <a href=' +
   '"https://kurtkaiser.us/">Kurt Kaiser</a> </p>';
  return submission;
}

// Check if sidebar user email option, add user email from sheet if it is
function checkUserEmail(properties) {
  Logger.log('in user email check');
  if (properties.userEmailRadio == 'onUserEmailRadio' && properties.emailsString) {
    properties.emailsString = properties.emailsString + ',' + 
        sheet.getRange(lastRow, parseInt(properties.userEmailColumn)).getValue();
  }
  return properties.emailsString;
}

// Send the email to the required parties
function emailSend(properties, htmlEmail, allEmails) {
  Logger.log('in emailSend');
  MailApp.sendEmail({
    to: allEmails, 
    subject: properties.emailSubject,
    htmlBody: htmlEmail
  })
}

Email.html

<!--
UI Sheets Email Notifications
Coded by Kurt Kaiser
kurtkaiser.us
All Rights Reserved, 2019
-->
<!DOCTYPE html>
<html>

<head>
  <style>
    body{
      text-align: center;
    }

    .box {
      text-align: center;
      font-family: Arial;
      width: 300px;
      border: 2px ridge blue;
      margin: 8px;
      margin-left: auto;
      margin-right: auto;
      padding: 10px;
      border: 3px outset rgba(0, 0, 0, 0.8);
      box-shadow: 4px 3px 19px 7px rgba(0, 0, 0, 0.55);
      height: 100%;
    }
    
    img {
      width: 100px;
    }

    #header {
      background-color: #ececec;
      /* gray: #ececec; blue: #d2e7d6  #e8f4ea*/
    }

    #content {
      background-color: white;
      margin-right: auto;
      margin-left: auto;     
    }

    #button {
      -webkit-border-radius: 28;
      -moz-border-radius: 5;
      border-radius: 5px;
      font-family: Arial;
      color: #ffffff;
      font-size: 15px;
      background: #ff7878;
      padding: 8px 20px 8px 20px;
      text-decoration: none;
    }
    
     #info{
        font-size: 14px;
      }
      
      #label{
        font-size: 15px;
        font-weight: bold;
      }
  </style>
  <base target="_top">
</head>
<body >
  <div class="box" id="header">
    <img src=
    "https://upload.wikimedia.org/wikipedia/commons/thumb/a/aa/Bullhorn_font_awesome.svg/512px-Bullhorn_font_awesome.svg.png" 
    width="180" style="margin:10px 0px"><br />
    <h2><?= messageTitle ?></h2>
  </div>
  <div id="content" class="box" >
    <h4>
      <?= messageBody ?>
    </h4>
  </div>
</body>

</html>

Form.html

<!DOCTYPE html>
<!--
UI Sheets Email Notifications
Coded by Kurt Kaiser
kurtkaiser.us
All Rights Reserved, 2019
-->

<html>

  <head>
  <!-- Style elements of the sidebar -->
   <style>
    
    html {
      background: #bbdcf0;
      color: black;
      line-height: 1.5;
      font-family: Verdana;
    }
    
    h2 {
      -webkit-text-stroke: 1px black;
      text-shadow: 2px 2px 0 #dadae3;
      margin-bottom: -4px;
    }
    
     #dataInput {
      background: #f1f2ee;
      padding: 5px;
      border-radius: 15px;
      margin: 5px;
      border-style: solid;
      border-width: 2px;
      border-color: #c3d5d8;
    }

    #instructions {
      font-size: 12px;
      margin-left: 10px;
      margin-right: 20px;
    }

    #info {
      font-weight: bold;
    }

    .myButton {
      background-color: #428bca;
      -moz-border-radius: 15px;
      -webkit-border-radius: 8px;
      border-radius: 15px;
      border: 1px solid #0c000c;
      display: inline-block;
      cursor: pointer;
      color: #ffffff;
      font-family: Arial;
      font-size: 16px;
      padding: 4px 15px;
      text-decoration: none;
      text-shadow: 0px 1px 0px #2f6627;
      margin: 5px;
    }

    .directions {
      font-size: 13px;
      margin-top: -2px;
      margin-bottom: -2px;
    }

    .myButton:hover {
      background-color: #94C3EA;
    }

    .myButton:active {
      position: relative;
      top: 1px;
    }

    input[type=text],
    textarea {
      width: 75%;
      padding-left: 5px;
      font-size: 14px;
    }
  </style>
    <title>Input Info</title>
  </head>

  <body>
    <center>
      <h2>Input Information</h2>
      <form>
        <div id='dataInput'>
          <div id='info'> Email Subject </div>
          <textarea rows="2"  id="emailSubject" name="emailSubject"></textarea>
          <span id='emailSubjectBox'></span>
        </div>
        <div id='dataInput'>
          <div id='info'> Message Title </div>
          <input type="text" id="messageTitle" name="messageTitle" />
          <span id='messageTitleBox'></span>
        </div>

        <div id='dataInput'>
          <div id='info'>Message</div>
          <textarea rows="4"  id="messageBody" name="messageBody">A new form has been submitted.</textarea>
          <span id='messageTitleBox'></span>
        </div>

        <div id='dataInput'>
          <div id='info'> Email User</div>
          <p class="directions">Should an email also be sent to the person who submits the form?</p>
          <input type="radio" onClick="onUserEmail()" id="onUserEmailRadio" value="onUserEmailRadio" name="userEmailRadio">Yes
          <input type="radio" onClick="offUserEmail()" id="offUserEmailRadio" checked="checked" value="offUserEmailRadio" name="userEmailRadio">No
          <span id='userEmailBox'></span>
        </div>

        <div id="dataInput">
          <div id="info">Number of Recipients</div>
          <!------- Loads from script -------->
          <span id="numOfRecipientsBox"></span>
        </div>

        <div id='dataInput'>
          <div id='info'>Emails</div>
          <span id='emailBox'> <!--1:<input type="text" id="email1" name="email1"/>--></span>
        </div>
        <div id='dataInput'>
          <div id='info'> Button Link</div>
          <p class="directions">
          A button linked to this spreadsheet will appear at the bottom of each email.
          </p>
          <input type="radio" id="onButton" value="onButton" name="buttonRadio">Yes
          <input type="radio" id="offButton" checked="checked" value="offButton" name="buttonRadio">No
          <span id='buttonBox'></span>
        </div>

        <div id="submitButton"></div>
        <div class="myButton" onClick="formSubmit()">Save</div>
        <div class="myButton" onClick="reload()">Reload</div>
        <div id="savedProperties"></div>
      </form>
      <br><p style="font-size:12px; text-align:right;">Created by
    <a href="https://kurtkaiser.us/">Kurt Kaiser</a> </p>
    </center>
  </body>

<!--------------------------------- Code ------------------------------------------->
  <script type="text/javascript">
    var numDropMenu = returnNumberMenu();
    var letterDropMenu = returnLetterMenu();

    google.script.run.withSuccessHandler(updateFormValues).getAllProperties();

    document.getElementById('numOfRecipientsBox').innerHTML =
      '<select id="numToEmail" value="1" name="numToEmail">' + numDropMenu;
      
        document.getElementById('columnsBox').innerHTML =
      'Start:<select id="startColumn" name="startColumn">' + letterDropMenu +
      '&nbsp;&nbsp;&nbsp;&nbsp;End:<select id="endColumn" name="endColumn">' + letterDropMenu;


    // ------------------------------------  Updating Form Values ----------------------------------
    function updateFormValues(properties) {
      document.getElementById('emailSubject').value = properties.emailSubject || 'Submission Notification';
      document.getElementById('messageTitle').value = properties.messageTitle || 'New Submission';
      document.getElementById('messageBody').value = properties.messageBody || 'A new form has been submitted.';
      document.getElementById('numToEmail').value = properties.numToEmail || '1';
      document.getElementById('emailBox').innerHTML = properties.newEmailBoxes  || 
           '1:<input type="text" id="email1" name="email1"/>';
      document.getElementById(properties.userEmailRadio).checked = true;     
     if (properties.userEmailRadio  == "onUserEmailRadio") {
        onUserEmail();
        document.getElementById('userEmailColumn').value = properties.userEmailColumn || "";
      }
      document.getElementById(properties.buttonRadio).checked = true; 
      }

    function formSubmit() {
      google.script.run.userInput(document.forms[0]);
      document.getElementById('submitButton').innerHTML = "<div id='dataInput'>Saved</div>";
      google.script.run.withSuccessHandler(hideSaved).waitSeconds();
    }
    
    function onUserEmail() {
      document.getElementById('userEmailBox').innerHTML =
        '<hr>Email Address Column <select id="userEmailColumn" name="userEmailColumn">' + letterDropMenu;
    }
    
    function offUserEmail() {
      document.getElementById('userEmailBox').innerHTML = '';
    }

    function hideSaved() {
      document.getElementById('submitButton').innerHTML = "";
    }

    function reload() {
      google.script.run.showFormSidebar();
    }
    
    function returnNumberMenu() {
    return '<option id="1" value="1">1</option><option value="2">2</option><option value="3">3</option>' +
      '<option value="4">4</option><option value="5">5</option><option value="6">6</option><option value="7">7</option>' +
      '<option value="8">8</option><option value="9">9</option><option value="10">10</option><option value="11">11</option>' +
      '<option value="12">12</option><option value="13">13</option><option value="14">14</option><option value="15">15</option>' +
      '<option value="16">16</option><option value="17">17</option><option value="18">18</option>' +
      '<option value="19">19</option><option value="20">20</option><option value="21">21</option><option value="22">22</option>' +
      '<option value="23">23</option><option value="24">24</option><option value="25">25</option></select>';
  }

  function returnLetterMenu() {
    return '<option id="1" value="1">A</option><option value="2">B</option><option value="3">C</option>' +
      '<option value="4">D</option><option value="5">E</option><option value="6">F</option><option value="7">G</option>' +
      '<option value="8">H</option><option value="9">I</option><option value="10">J</option><option value="11">K</option>' +
      '<option value="12">L</option><option value="13">M</option><option value="14">N</option><option value="15">O</option>' +
      '<option value="16">P</option><option value="17">Q</option><option value="18">R</option>' +
      '<option value="19">S</option><option value="20">T</option><option value="21">U</option><option value="22">V</option>' +
      '<option value="23">W</option><option value="24">X</option><option value="25">Y</option><option value="26">Z</option>' +
      '</select>';
  }

  </script>

</html>