Who's a Necessary Party?

A necessary party is one who is not indispensable but ought to be joined as a party if complete relief is to be accorded as to those already parties, or for a complete determination or settlement of the claim subject of the action.
A necessary party ought to be joined as a party if complete relief is to be accorded as to those already parties (Sec. 8, Rule 3). The non-inclusion of a necessary party does not prevent the court from proceeding in the action, and the judgment rendered therein shall be without prejudice to the rights of such necessary party (Sec. 9, Rule 3).