Rule 20.Permissive joinder of parties

Rule 20(a) sets the test for joining people who were not required to be joined under Rule 19. Multiple plaintiffs may join in one action if they assert a right to relief, jointly, severally, or in the alternative, arising out of the same transaction, occurrence, or series of transactions or occurrences, and if some question of law or fact common to all of them will come up in the case. The same test runs in reverse for defendants: they may be joined together if a right to relief is asserted against them arising from the same transaction or series of transactions, with a common question running through the claims. Neither a joined plaintiff nor a joined defendant needs to be interested in every bit of relief sought, and the court can enter judgment for some plaintiffs and against some defendants according to each one's own rights and liabilities, without treating the group as all-or-nothing.

Section (b) gives the court a check on that flexibility. When including a party would embarrass, delay, or put another party to expense -- particularly a party against whom no claim is asserted and who asserts no claim in return -- the court may enter orders to prevent that, including ordering separate trials or other measures to avoid delay or prejudice.