I have no idea what the etiquette is here. Our friend, a male, is in a relationship with two women. One is his wife and one is his gf. They all live together as far as I know. I hadn't included the gf in the guest count, because we don't know her (we don't know the wife either) and I hadn't thought much about it. Do we consider the 3 of them a social unit and invite all? What would you do?