In that particular circumstance, courts have been divided on which rule to apply. Admittedly, as the Eastern District of New York observed, there is "little practical difference between Rule 15 and Rule 21 since they both leave the decision whether to permit or deny an amendment to the district court's discretion." Amaya v. Roadhouse Brick Oven Pizza, Inc., 285 F.R.D. 251, 253 (E.D.N.Y. 2012). And so courts have actually granted motions to amend that add or subtract parties pursuant to both Rule 15 and Rule 21. See Hernandez v. Sikka, No. 17CV4792SJFSIL, 2019 WL 1232092, at *5 (E.D.N.Y. Mar. 15, 2019).
In Hernandez v. Sikka, the court determined that, where a defendant allegedly served as the plaintiff's "employer," the potential defendant could be subject to personal liability in connection with the plaintiff's claims, and there existed no evidence of bad faith on the part of the plaintiffs, the defendant could be added to the complaint via amendment pursuant to both Fed. R. Civ. P. 15(a) and 21.
In Hernandez v. BMNY Contracting Corp., the court chose to employ Rule 21, rather than Rule 15, when deciding the plaintiff's request to add new parties without asserting new claims. No. 17 CIV. 9375 (GBD), 2019 WL 418498, at *1 (S.D.N.Y. Jan. 17, 2019). However, the court noted that while it was applying Rule 21, "the consensus in this [district] is that when 'deciding whether to allow joinder, courts adhere to the same standard or liberality afforded to motions to amend [pleadings] under Rule 15.'" This disposition only furthered the idea that the two rules, while sometimes applied differently in different circumstances, are intertwined, and exceedingly similar.
Together, Rules 15 and 21 provide movants with an extremely liberal standard to meet. Courts should generally grant motions to amend absent any evidence of bad faith, undue delay, or undue prejudice to the non-movant. Quaratino v. Tiffany & Co., 71 F.3d 58, 66 (2d Cir. 1995). Nevertheless, attorneys should remember that, while the standards are exceedingly liberal, a motion to amend under either Rule 15 or Rule 21 should be expedient and logically sound to ensure a favorable judicial decision.