Moving a field into a (different) group/fieldset using hook_form_alter()

The following demonstrates how to move a field into a different fieldset within a form using hook_form_alter() and a special #after_build form property.

<?php
function mymodule_form_alter(&$form, $form_state, $form_id) {
  if ($form_id == 'status_node_form' && $form['nid']['#value']) {
    $form['send_notification'] = array(
      '#type' => 'submit',
      '#value' => t('Send Notification'),
      '#submit' => array('mymodule_notify_submit'),
    );
    $form['#after_build'][] = 'mymodule_notification_afterbuild';
  }
}

function mymodule_notification_afterbuild($form, $form_state) {
  // Add the notifications button to the "Notifications" field group
  // (defined in the content type field manager)
  $form['group_notifications']['send_notification'] = $form['send_notification'];
  unset($form['send_notification']);
  return $form;
}
?>