Section 641 - Joinder of parties needed for just adjudicationA person shall be joined as a party in the action when either:
(1) In his absence complete relief cannot be accorded among those already parties.(2) He claims an interest relating to the subject matter of the action and is so situated that the adjudication of the action in his absence may either:(a) As a practical matter, impair or impede his ability to protect that interest.(b) Leave any of the persons already parties subject to a substantial risk of incurring multiple or inconsistent obligations.