Mont. Code Ann. § 27-8-301

When declaratory relief is sought, all persons shall be made parties who have or claim any interest which would be affected by the declaration, and no declaration shall prejudice the rights of persons not parties to the proceeding. In any proceeding which involves the validity of a municipal ordinance or franchise, such municipality shall be made a party and shall be entitled to be heard, and if the statute, ordinance, or franchise is alleged to be unconstitutional, the attorney general of the state shall also be served with a copy of the proceeding and be entitled to be heard.