public function get_submission_group($userid) {

        if (isset($this->usersubmissiongroups[$userid])) {
            return $this->usersubmissiongroups[$userid];
        }

        $groups = $this->get_all_groups($userid);
        if (count($groups) != 1) {
            $return = false;
        } else {
            $return = array_pop($groups);
        }

        // Cache the user submission group.
        $this->usersubmissiongroups[$userid] = $return;

        return $return;
    }