2015 New York Laws
CVP - Civil Practice Law & Rules
Article 10 - (Civil Practice Law & Rules) PARTIES GENERALLY
1013 - Intervention by permission.

 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.