803.03.Joinder of persons needed for just and complete adjudication.

Someone subject to service of process must be joined if complete relief cannot be given among the current parties without them, or if that person claims an interest in the subject of the action and their absence could, as a practical matter, impair their ability to protect that interest or leave existing parties facing a substantial risk of double, multiple, or inconsistent obligations. A related set of rules covers subrogated, derivative, and assigned claims: a party pursuing a principal claim must join anyone who, at the time the action starts, holds a subrogated, derivative, or assigned piece of that claim, including a right to recover for loss of consortium and certain public assistance providers with subrogation rights. Once joined that way, a person can participate in prosecuting the case, waive participation and let the joining party represent its interest, subject to a signed waiver, or move to dismiss its own claim without prejudice if being forced to litigate jointly would be unjust, with the court weighing the possible prejudice against the state’s interest in judicial economy.

When a necessary person cannot be joined, the court weighs whether a judgment rendered in that person’s absence would prejudice them or the existing parties, whether protective measures could lessen that prejudice, whether such a judgment would be adequate, and whether the plaintiff would have an adequate remedy if the case were dismissed instead, in order to decide whether the case should proceed or be treated as one where the absent person is indispensable.

A pleading asserting a claim for relief must name any known, unjoined necessary persons and explain why they were not joined. And section 803.03 itself gives way to the separate joinder scheme for class actions in section 803.08.