N.Y. C.P.L.R. § 1003
Nonjoinder of a party who should be joined under section 1001 is a ground for dismissal of an action without prejudice unless the court allows the action to proceed without that party under the provisions of that section. Misjoinder of parties is not a ground for dismissal of an action. Parties may be added at any stage of the action by leave of court or by stipulation of all parties who have appeared, or once without leave of court within twenty days after service of the original summons or at anytime before the period for responding to that summons expires or within twenty days after service of a pleading responding to it. Parties may be dropped by the court, on motion of any party or on its own initiative, at any stage of the action and upon such terms as may be just. The court may order any claim against a party severed and proceeded with separately.
N.Y. CPLR 1003