Pendent party jurisdiction is the power of a court to hear a case involving a party or parties who wouldn't ordinarily fall under its purview. A court only has this authority under limited circumstances. Specifically, it has this type of jurisdiction when a secondary issue involving the party or parties stems from a matter that the court already has the power to hear.
The U.S. Supreme Court addressed the issue of pendent party jurisdiction in Finley v. United States, 490 U.S. 545 (1989). The Supreme Court ruled that there was not sufficient reason to permit a federal court to hear a case in which an aggrieved party wanted to combine the claims against a California municipality, utility and the federal government, simply because they all stemmed from the same incident.
However, Congress intervened and created rules permitting pendent party jurisdiction in certain types of cases.