<?php
namespace Drupal\bulk_email\Form;
use Drupal\Core\Form\FormBase;
use Drupal\Core\Form\FormStateInterface;
use Drupal\Component\Utility\UrlHelper;

class EmailForm extends FormBase {
  public function getFormId() {
    return "bulk_email_form";
  }

  public function buildForm(array $form, FormStateInterface $form_state) {
    $form['#tree'] = TRUE;

    $form['users'] = array(
      '#type' => 'fieldset',
      '#title' => t('Users'),
      '#prefix' => '<div id="users-wrapper">',
      '#suffix' => '</div>',
    );
    $form['temptest'] = array(
      '#type' => 'textBox

    $form['num_users'] = array(
      '#type' => 'textfield',
      '#title' => 'test',
      '#description' => 'enter anything to add a user',
    );

    $num_users = $form_state->getValue('num_users');
    if (!empty($form_state->getValue("temptest"))) {
      $num_users++;
      $form_state->setValue('num_users', $num_users);
      $form_state->setRebuild(TRUE);
    }
    if (empty($num_users)) {
      $num_users = 1;
    }
    for ($i = 0; $i < $num_users; $i++) {
      $form['users'][$i] = array(
        '#type'  => 'textfield',
        '#description' => t('Select a user.'),
      );
    }

    $form['users']['users_more'] = array(
        '#type' => 'submit',
        '#value' => t('Add one'),
      '#submit' => array($this, 'bulk_email_addfieldsubmit'),
        '#ajax' => array(
               'callback' => array($this, 'bulk_email_add_more_callback'),
           'wrapper' => 'users-wrapper',
      ),
     );

   $form['actions'] = array('#type' => 'actions');
    $form['submit'] = array(
      '#type' => 'submit',
      '#value' => t('Send'),
    );
    return $form;
  }

  public function submitForm(array &$form, FormStateInterface $form_state) {
    foreach ($form_state->getValues() as $key => $value) {
      drupal_set_message($key . ': ' . $value);
    }
  }

  public function bulk_email_addfieldsubmit(array &$form, FormStateInterface $form_state) {
    $num_users = $form_state->getValue('num_users') + 1;
    $form_state->setValue('num_users', $num_users);
    $form_state->setRebuild(TRUE);
  }

  public function bulk_email_add_more_callback(array &$form, FormStateInterface $form_state) {
    return $form['users'];
  }
}
?>