<?php
define( '_JEXEC', 1 );
define('JPATH_BASE', __DIR__."/../.." );
define( 'DS', DIRECTORY_SEPARATOR );
/* Required Files */
require_once ( JPATH_BASE .DS.'includes'.DS.'defines.php' );
require_once ( JPATH_BASE .DS.'includes'.DS.'framework.php' );
$language = JFactory::getLanguage();
$language->load('com_users', JPATH_BASE);
JFactory::getLanguage()->load('mod_login', JPATH_BASE, 'de-DE', true);

$app = JFactory::getApplication('site');
$app->initialise();
require_once(JPATH_BASE.DS.'components'.DS.'com_users'.DS.'models'.DS.'registration.php');
//not necessary
// require_once(JPATH_BASE.DS.'libraries'.DS.'joomla'.DS.'application'.DS.'component'.DS.'helper.php');
$model = new UsersModelRegistration();
jimport('joomla.mail.helper');
jimport('joomla.user.helper');

$username = 'TestUser';
$name = 'TestName';
$email = 'test@test.de';
$password = 'test';
$data = array(
    'username' => $username,
    'name' => $name,
    'email1' => $email,
    'password1' => $password, // First password field
    'password2' => $password, // Confirm password field
    'block' => 0 );
echo $model->register($data);
?>