acting party (Epstein) who has not been made a party to the proceedings before the Court. See, e.g., School Dist of City of Pontiac v. Secretary of U.S. Dept of Ethic., 584 F.3d 253, 303 (6th Cir. 2009) ("It is hornbook law that all parties to a contract are necessary in an action challenging its validity . . . ."