Drupal Registration Module Email Registrants tab
Not being a developer I had to figure this out using hook_form_alter. Too some digging and a little help.

function ecg_registration_form_registration_registrations_broadcast_form_alter(&amp;$form, &amp;$form_state) {
  //dpm($form);
  $form['subject'] = array(
    '#type' =&gt; 'textfield',
    '#title' =&gt; t('Subject'),
    '#description' =&gt; t('The subject of the message.'),
    '#required' =&gt; TRUE,
    '#size' =&gt; 80,
    '#maxlength' =&gt; 80,
  );
}


Hope that helps someone.