function sendInvites_(user, response) {
  let id = ScriptProperties.getProperty('calId');
  let cal = CalendarApp.getCalendarById(id);
  for (let i = 0; i < response.length; i++) {
    cal.getEventSeriesById(response[i][5]).addGuest(user.email);
  }
}
Add the user as a guest for every session he or she selected.
@param {object} user An object that contains the user's name and email.
@param {Array<String[]>} response An array of data for the user's session choices.