public function getMembers($force = false)
    {
        if ($this->nb_user_group_member_list->isEmpty() || $force) {
            $this->nb_user_group_member_list->clear();
            if ($this->isValueNumeric('nb_user_group_id')) {
                $this->nb_user_group_member_list->merge(CNabuUserGroupMember::getMembersOfGroup($this));
            }
        }

        return $this->nb_user_group_member_list;
    }