Добавить пользователя с помощью API Joomla можно так:

require_once(JPATH_ROOT . DIRECTORY_SEPARATOR . 'components' . DIRECTORY_SEPARATOR . 'com_users' . DIRECTORY_SEPARATOR . 'models' . DIRECTORY_SEPARATOR . 'registration.php');

$user = new JUser;

$userData = array(
 'name'=>'name',
 'username'=>'username',
 'password'=>'111111',
 'password2'=>'111111',
 'email'=>'email@pochta',
 'id'=>0,
 'groups'=>Array(2)
);

if (!$user->bind($userData)) {
 $this->setError(JText::sprintf('COM_USERS_REGISTRATION_BIND_FAILED', $user->getError()));
 return false;
}

if (!$user->save()) {
 $this->setError(JText::sprintf('COM_USERS_REGISTRATION_SAVE_FAILED', $user->getError()));
 return false;
}