Massachusetts General Laws Part III. Courts, Judicial Officers and Proceedings in Civil Cases (Ch. 211-262) Ch. 241, § 9

If any party named in the petition has not been served personally with the citation and has not appeared, or is a minor or under other disability and has no guardian or other legal representative within the commonwealth, or if any person whose name is unknown, or who is unascertained or not in being, appears by the record in the case to have an estate, vested or contingent, as a co-tenant of the land of which partition is sought, the court shall appoint a suitable person to act for him in the proceedings.