Celebrity Cocktail Party Attire?

My local theater group will be putting on Thoroughly Modern Millie in mid June. Apparently Jennifer Garner, who is from the area, will be coming to see us and have a benefit cocktail reception afterward. The cast is supposed to go, but I don't know what I'm supposed to wear: I've never been to a cocktail party before, much less one hosted by a celebrity. Cast members get in free, but anyone else has to pay $100 to attend, if that information helps anyone. Thanks for your time.