public static function get_group_members($groupids) {
        $members = array();

        $params = self::validate_parameters(self::get_group_members_parameters(), array('groupids'=>$groupids));

        foreach ($params['groupids'] as $groupid) {
            // validate params
            $group = groups_get_group($groupid, 'id, courseid, name, enrolmentkey', MUST_EXIST);
            // now security checks
            $context = context_course::instance($group->courseid, IGNORE_MISSING);
            try {
                self::validate_context($context);
            } catch (Exception $e) {
                $exceptionparam = new stdClass();
                $exceptionparam->message = $e->getMessage();
                $exceptionparam->courseid = $group->courseid;
                throw new moodle_exception('errorcoursecontextnotvalid' , 'webservice', '', $exceptionparam);
            }
            require_capability('moodle/course:managegroups', $context);

            $groupmembers = groups_get_members($group->id, 'u.id', 'lastname ASC, firstname ASC');

            $members[] = array('groupid'=>$groupid, 'userids'=>array_keys($groupmembers));
        }

        return $members;
    }