Well my character is a glorified bodyguard as well as a wellspring of information on quite a few illegal operations through his informants and the prostitutes he lives with.
He would have a very good reason for being at the party, either as someones bodyguard (could be another character's), his girls protector, or to deliver information.

If anyone wants to use him for their characters bodyguard then they can do. It would also have one relationship established, so a couple of character would actually know each other without having to be related.