//внутри функции usf_edit_users_query() заменяем все $_GET на $_REQUEST

//список пользователей оборачиваем div-ом с классом rcl-userlist

//тут вешаем обработчик на onclick
$content = rcl_get_form(array(
        'method' => 'post',
        'submit' => __('Сохранить'),
        'onclick' => 'my_send_form_data(\'my_ajax_get_userlist\', this);return false;',
        'fields' => $fields
    ));

//скрипт js
function my_send_form_data(action,e){

    var form = jQuery(e).parents('form');

    if(!rcl_check_form(form)) return false;

    if(e && jQuery(e).parents('.preloader-parent')){
        rcl_preloader_show(jQuery(e).parents('.preloader-parent'));
    }

    rcl_ajax({
        data: form.serialize() + '&action=' + action,
        success: function(data){
           jQuery('#rcl-tabs .rcl-userlist').replaceWith(data.content);
        }
    });

}

//ajax-обработчик
rcl_ajax_action('my_ajax_get_userlist', false);
function my_ajax_get_userlist(){
    global $rcl_user,$rcl_users_set,$user_ID;

    require_once RCL_PATH.'classes/class-rcl-users-list.php';

    $users = new Rcl_Users_List(array('template'=>'rows'));

    $count_users = false;

    if(!isset($atts['number'])){

        $count_users = $users->count();

        $id_pager = ($users->id)? 'rcl-users-'.$users->id: 'rcl-users';

        $pagenavi = new Rcl_PageNavi($id_pager,$count_users,array('in_page'=>$users->query['number']));

        $users->query['offset'] = $pagenavi->offset;
    }

    //тут изменяется query
    usf_edit_users_query($users->query);

    $usersdata = $users->get_users();

    $userlist = $users->get_filters($count_users);

    //список пользователей оборачиваем div-ом с классом rcl-userlist
    $userlist .= '<div class="rcl-userlist">';

    if(!$usersdata){
        $userlist .= '<p align="center">'.__('Users not found','wp-recall').'</p>';
    }else{

        if(!isset($atts['number']) && $pagenavi->in_page)
            $userlist .= $pagenavi->pagenavi();

        $userlist .= '<div class="userlist '.$users->template.'-list">';

        $rcl_users_set = $users;

        foreach($usersdata as $rcl_user){ $users->setup_userdata($rcl_user);
            $userlist .= rcl_get_include_template('user-'.$users->template.'.php');
        }

        $userlist .= '</div>';

        if(!isset($atts['number']) && $pagenavi->in_page)
            $userlist .= $pagenavi->pagenavi();

    }

    $userlist .= '</div>';

    $users->remove_filters();

    wp_send_json(array(
        'content' => $userlist
    ));
}