New York Consolidated Laws, Civil Practice Law and Rules - CVP § 1013. Intervention by permission

Upon timely motion, any person may be permitted to intervene in any action when a statute of the state confers a right to intervene in the discretion of the court, or when the person's claim or defense and the main action have a common question of law or fact. In exercising its discretion, the court shall consider whether the intervention will unduly delay the determination of the action or prejudice the substantial rights of any party.