Even if someone came up to me after a gig and said, "Hey, I''m a perfect stranger but come to my party", I'd want to be sure that people I know are going along. I'd be even more comfortable if I was sure that people I know are friends with the person in question, and can assure me that he's an "okay guy" and that nothing will happen at his party to make me uncomfortable (or to prompt a visit from the police, or anything!). A girl can't be too careful these days!