Drupal - resend welcome e-mails on users administration form.

resend.php

<?php

/**
 * Implements hook_user_operations().
 */
function example_user_operations() {
  $operations = array(
    'example_resend_welcome_email' => array(
      'label' => t('Re-send welcome e-mail'),
      'callback' => 'example_resend_welcome_email',
    )
  );
  return $operations;
}

/**
 *
 */
function example_resend_welcome_email($accounts) {
  $accounts = user_load_multiple($accounts);
  foreach ($accounts as $account) {
    watchdog('example', 'Re-sent welcome e-mail to %mail.', array('%mail' => $account->mail));
    drupal_set_message(t('Re-sent welcome e-mail to %mail', array('%mail' => $account->mail)));
    $op = 'register_admin_created'; // @see https://api.drupal.org/api/drupal/modules!user!user.module/function/_user_mail_notify/7
    _user_mail_notify($op, $account); 
  }
}