15 U.S.C. § 5 : US Code - Section 5: Bringing in additional parties

      Whenever it shall appear to the court before which any proceeding
    under section 4 of this title may be pending, that the ends of
    justice require that other parties should be brought before the
    court, the court may cause them to be summoned, whether they reside
    in the district in which the court is held or not; and subpoenas to
    that end may be served in any district by the marshal thereof.