What is the test for a motion to amend a notice of complaint to add a proper party to the original complaint?

We first note that the "principal function of procedural rules should be to serve as useful guides to help, not hinder, persons who have a legal right to bring their problems before the courts," and "decisions on the merits are not to be avoided on the basis of mere technicalities." Schiavone v. Fortune, 477 U.S. 21, 27, 106 S.Ct. 2379, 2383, 91 L.Ed.2d 18 (1986) (cites omitted) (nonetheless holding that because notice of the original complaint was missing, amendment to add the proper party defendant was barred).