craft()->on('userGroups.onAssignUserToGroups',function(Event $event){
    $user_id = $event->params['userId'];
    $user = craft()->users->getUserById($user_id);
    $userId = craft()->request->getPost('userId');
    $isNewUser = !$userId;

    $email = new EmailModel();
    $emailSettings = craft()->email->getSettings();
    $email->fromEmail = $emailSettings['emailAddress'];
    $email->sender = $emailSettings['emailAddress'];

    if ($isNewUser){
    /* generate random password */
    $randomPassword = $this->getRandomString(8,false);
    /* set random password to user */
    $user->newPassword = $randomPassword;
    if (craft()->users->saveUser($user)){
             $result = "success";
         }else{
             $result = "fail";
              }
    $email->subject = 'confirm created email'
    $email->body = "Text to user".$result;
    $email->toEmail = $user->email;
    craft()->email->sendEmail($email);
   }
});