Send pending approval mails to all users who have the 'administer users' permission

mymodule.php

<?php

/**
 * Implements hook_mail_alter().
 *
 * Also send pending approval mails to users who have
 * the 'administer users' permission
 *
 * @see _user_mail_notify()
 */
function mymodule_mail_alter(&$message) {
  if ($message['id'] == 'user_register_pending_approval_admin' && !isset($message['params']['stop_propagation'])) {
    $message['params']['stop_propagation'] = TRUE;
    $roles = user_roles(TRUE, 'administer users');
    $uids = db_select('users_roles', 'ur')
      ->fields('ur', array('uid'))
      ->condition('rid', array_keys($roles))
      ->execute()
      ->fetchCol();
    $users = user_load_multiple($uids);
    foreach ($users as $account) {
      drupal_mail('user', 'register_pending_approval_admin', $account->mail,user_preferred_language($account), $message['params']);
    }
  }
}