Section 31. The court may appoint a suitable person to appear and act in said proceedings as guardian ad litem for minors and for all persons and classes or descriptions of persons under disability or not ascertained or unknown or not in being or out of the commonwealth, and who under the provisions of the will in regard to said trust estate or otherwise may be or may become interested in said trust estate, and the court shall appoint a suitable person to appear and act therein as guardian ad litem of said absentee. An order or decree in such proceedings, made after such appointment, shall be conclusive upon all persons for whom such guardian ad litem was appointed.