Dynamic list options in Google Forms.

code.gs

//Any option added in other by a user who submits the form is then avaialbe in the explicit options for other users.
//This by default acts on all checkbox and multiple choice responses in the form. Make sure each has at least one option
//other than "other" to begin with or else you're gonna have a bad time.

function onSubmit(e) {
  var formResponse = e.response;
  var itemResponses = formResponse.getItemResponses();
  var item, itemAsItemType, choices, choicesStrings, io, response, a, b;
  
  for (a = 0; a < itemResponses.length; a++) {
    response = itemResponses[a].getResponse();
    if (response === '') {
      //bogey, bail
      Logger.log('Empty Response, something wrong here.');
      return;
    }
    
    item = itemResponses[a].getItem();
    switch (item.getType()) {
      case FormApp.ItemType.CHECKBOX:
        itemAsItemType = item.asCheckboxItem();
        break;
      case FormApp.ItemType.MULTIPLE_CHOICE:
        itemAsItemType = item.asMultipleChoiceItem();
        break;
      default:
        return;
    }
    
    choices = itemAsItemType.getChoices();
    choicesStrings = [];
    for (b = 0; b < choices.length; b++) {
      choicesStrings[b] = choices[b].getValue();
    }
    
    if (Array.isArray(response)) {
      for (b = 0; b < response.length; b++) {
        io = choicesStrings.indexOf(response[b]);
        if (io === -1) {
          Logger.log('Missing choice "%s". Adding to available choices.', response[b]);
          choicesStrings.push(response[b]);
        }
      }
    } else {
      io = choicesStrings.indexOf(response);
      if (io === -1) {
        console.log('Missing choice "%s". Adding to available choices.', response);
        choicesStrings.push(response);
      }
    }
    //need to clear 'other' option before setting choices or it will throw an error.
    //Also can't clear other option if form has no current options so if you don't
    //seed the question with at least one option the item will be caught in a position
    //that cannot be corrected by code and only through the editor web interface.
    itemAsItemType.showOtherOption(false);
    itemAsItemType.setChoiceValues(choicesStrings);
    itemAsItemType.showOtherOption(true);
  }
}