Fed. R. Civ. P. 20(a)
Definition
A rule of the Federal Rules of Civil Procedure establishing the circumstances under which a party may join a case as a plaintiff or be joined to the case as a defendant, both of which generally require that there be issues of law or fact common to all parties and that a party to be joined share with the initial plaintiff or defendant, as appropriate, a right of recovery or potential for liability arising out of the same event or transaction.