Show a selectable list of users

frontend-select-users.php

<?php 
    $users = get_users( array( 'fields' => array( 'ID' ) ) );
    $html[] .= "<select id='event-users' multiple>";
    foreach($users as $user_id){
        if ( get_current_user_id() != $user_id->ID ) {
            $data = get_user_meta ( $user_id->ID );
            $html[] .= "<option value='$user_id->ID'>";
            $html[] .= $user_id->ID;
            $html[] .= " - ";
            $html[] .= $data['first_name'][0];
            $html[] .= " ";
            $html[] .= $data['last_name'][0];
            $html[] .= "</option>";
        }
    } 
    $html[] .= "</select>";
    $arr = implode("", $html);
    echo $arr;
?>