Loading a form via AJAX with drupal_get_form()

I am trying to load a form via ajax, here is the code i use:

JS:

bigPopupBut.click(function () {
        jQuery.ajax({
            url: "http://site/group_create",
            dataType: "html",
            success: function (data) {
                bigPopup.append(data);
            }
        });
})

The group_create is a PHP script that returns me a form, here is the code i used:

$node_type = 'group';
    $form_id = $node_type . '_node_form';     
    $node = new stdClass();
    $node->uid = $user->uid;
    $node->name = $user->name ;
    $node->field_city['und']['0']['value'] = $user->field_city['und']['0']['value'];
    $node->field_country['und']['0']['value'] = $user->field_country['und']['0']['value'];
    $node->type = $node_type;
    $node->language = LANGUAGE_NONE;
    $node->path = '';
    node_object_prepare($node);
    $return=drupal_get_form($form_id,$node);
return $return;

The Problem i have is that the PHP callback doesn't just return me the form, but a whole page, with headers, body, so when i append it with JS i get a duplicate of the whole page in my bigPopup DIV.

What am i doing wrong ? how could i just get the form without the surounding page?

Thanks all for the help.