Saturday night’s gala performance of “The Gershwins’ Porgy and Bess’’ didn’t just have a pre-party for V.I.P.’s, it had a post-party, too. The soirees, which raised money for the American Repertory Theater, were cochaired by ART trustees Kevin Costin and Ann Gund, who threw the events at the home of Kevin’s sister, RoAnn Costin, and her husband, Jim Bailey. It happened to be RoAnn’s birthday, so the “Porgy’’ cast treated her to a song. The guest list for the two-part benefit included Harvard provost Alan Garber, ART founding director Robert Brustein, Harvard president Drew Faust, Boston Symphony Orchestra managing director Mark Volpe, and event sponsors Ted Cutler, Steve and Rosemarie Johnson, and Katherine Chapman and Tom Stemberg.