Threesome!!!!! Only kidding!
Generally, If a girl asks you that it means she's either not sure if your a guy worth seeing, and she wants her friends advice or she sees you as a friend. You could invite a mate and then decide between you, who like whom or if you go it alone, be a fun, chill guy and have a great time.
Remember though, if you go on your own and decide early on, that you only like one girl DON'T just pay her attention all night!!! If you don't her friiend will either try and get her to go early or say nothing but then bad mouth you when they get home! Chat and laugh with the friend and when she goes to the toilet/gets a drink etc.. tell your girl that your having a great time but next time want to see her on her own, to get to know her more.
Hope it goes well!