Rule 19.Required joinder of parties.

Rule 19 identifies people who belong in a lawsuit even though no one has named them. A person who can be served and whose joinder would not strip the court of jurisdiction must be added as a party if, without that person, the court cannot grant complete relief to the parties already in the case, or if leaving that person out could impair their ability to protect their own interest or expose an existing party to double or inconsistent obligations. If such a person refuses to join as a plaintiff, the rule lets the court realign them as a defendant or an involuntary plaintiff instead, and a joined party who has a valid venue objection must be dismissed rather than forced to litigate in the wrong court.

Sometimes the person who should be joined cannot be — perhaps they cannot be served, or joining them would destroy the court's jurisdiction. When that happens, the court weighs whether to proceed among the existing parties or dismiss the case entirely, looking at how much a judgment might prejudice the absent person or the current parties, whether that prejudice can be softened through protective terms in the judgment or by shaping the relief differently, whether a judgment issued without the absent person would still be adequate, and whether the plaintiff would have anywhere else to turn if the case were dismissed.

Any complaint that names a claim must also flag the problem: if a person who should be joined is not, the pleading has to state that person's name, if known, and explain why they were left out. The rule yields to Rule 23, so it does not override how class actions handle absent class members.