Don't you think she wants to be with her friend to make sure you're not a creep/freak who's dangerous or something? That would be my thought process if I were a female okc'ing, but who knows.
Isn't that the point of meeting in public places? I don't know why someone would think it's a good idea for a first date to be them ganging up with their friends in a personal setting against you. Personally, I wouldn't have gone either. It's a real gamble for house parties if you don't know someone there.