For a large group Margarita and Mexican outing you really can't beat Cactus Cantina. Okay, for atmosphere, it's sister restaurant, Lauriol Plaza just north of Dupont, JUST might have it beat, but let's face it, the wait times are absolutely off the wall.
Cactus Cantina serves a similar menu with the same awesome margaritas and a much calmer, laid back atmosphere, perfect for relaxing with friends.
Try the seafood enchiladas (usually masquerading as the enchiladas de mariscos or something) or my absolute favorite, the crab and shrimp quesadillas, almost always on the daily special menu. If you're not a seafood lover, the cheese enchiladas are good, and I've never heard a complaint about the salads.