case "members":
        $members = array();
        foreach (Groups::get_by_id($_REQUEST["group"])->get_users() as $user) {
            $members[$user->get_id()] = $user->get_username();
        }
        for ($i == 1; $i <= 4; $i++) {
            unset($members[$i]);
        }
        // don't include system, root, nobody, guest
        echo json_encode($members);
        break;
    case "add-user":
        $group = Groups::get_by_id($_REQUEST["group"]);
        $user = Users::get_by_username($_REQUEST["user"]);
        if ($user) {
            $group->add_user($user);
            echo json_encode(array($user->get_id() => $user->get_username()));
        } else {
            echo json_encode(array(0, ""));
        }
        break;
    case "del-user":
        $group = Groups::get_by_id($_REQUEST["group"]);
        $user = Users::get_by_id($_REQUEST["user"]);
        if ($user) {
            $group->remove_user($user);
            echo json_encode(array($user->get_id() => $user->get_username()));
        } else {
            echo json_encode(array(0, ""));
        }
}