The Federal Rules of Civil Procedure
Required Joinder of Parties
(a) Persons Required to Be Joined if Feasible.
(1) Required Party. A person who is subject to service of process and whose joinder will not deprive the court of subject-matter jurisdiction must be joined as a party if:
(A) in that person's absence, the court cannot accord complete relief among existing parties; or
(B) that person claims an interest relating to the subject of the action and is so situated that disposing of the action in the person's absence may: (i) as a practical matter impair or impede the person's ability to protect the interest; or (ii) leave an existing party subject to a substantial risk of incurring double, multiple, or otherwise inconsistent obligations because of the interest.
(2) Joinder by Court Order. If a person has not been joined as required, the court must order that the person be made a party. A person who refuses to join as a plaintiff may be made either a defendant or, in a proper case, an involuntary plaintiff.
(3) Venue. If a joined party objects to venue and the joinder would make venue improper, the court must dismiss that party.
(b) When Joinder Is Not Feasible. If a person who is required to be joined if feasible cannot be joined, the court must determine whether, in equity and good conscience, the action should proceed among the existing parties or should be dismissed. The factors for the court to consider include:
(1) the extent to which a judgment rendered in the person's absence might prejudice that person or the existing parties;
(2) the extent to which any prejudice could be lessened or avoided by:
(A) protective provisions in the judgment;
(B) shaping the relief; or
(C) other measures;
(3) whether a judgment rendered in the person's absence would be adequate; and
(4) whether the plaintiff would have an adequate remedy if the action were dismissed for nonjoinder.
(c) Pleading the Reasons for Nonjoinder. When asserting a claim for relief, a party must state:
(1) the name, if known, of any person who is required to be joined if feasible but is not joined; and
(2) the reasons for not joining that person.
(d) Exception for Class Actions. This rule is subject to Rule 23.