Litigation structure depends on who is before the court, and several mechanisms control that.
Required joinder. A person must be joined if complete relief cannot be accorded among existing parties without them, or if they claim an interest and proceeding without them would impair that interest or leave an existing party exposed to inconsistent obligations. If joinder would destroy jurisdiction, the court decides in equity and good conscience whether to proceed or dismiss.
Permissive joinder of plaintiffs or defendants requires a right to relief arising from the same transaction or series of transactions and a common question of law or fact.
Counterclaims and crossclaims. Compulsory counterclaims arise from the same transaction and must be pleaded; permissive ones may be. Crossclaims between co-parties must arise from the same transaction.
Third-party practice. A defendant may bring in a person who is or may be liable to it for all or part of the claim — indemnity, contribution or warranty. It is not a vehicle for asserting that the third party rather than the defendant is liable to the plaintiff.
Intervention as of right requires a timely application, an interest in the subject matter, practical impairment of that interest, and inadequate representation by existing parties. Permissive intervention requires a common question and no undue delay or prejudice.
Interpleader protects a stakeholder facing competing claims to the same fund.