This is a murky issue. It's pushing all my etiquette-violations buttons, though.
My best friend, R, has adopted me into her pre-existing circle of friends to the point that I now have independent friendships with several of her friends. Every summer, a male friend of hers, T, has a cookout at his home (a 1.5 hour drive from our hometown). Now, I barely know T. In fact I find him annoying, although he's a nice enough guy. He's dating our friend P.
Two years running R has said "Oh, of course you're invited to T's party!" despite the fact that I (at that time) had barely ever met T and had not been invited by him. Now, it's an informal party with word-of-mouth kind of invitations, but I didn't feel quite right showing up to a party where I had no acknowledgment from the host that I was expected. I don't feel like I AM invited just because R assumes I am welcome.
Cut to this current year. I hadn't given the party a thought and hadn't heard anything about it (the party is tomorrow, 6/9/07) until last weekend, when P (T's girlfriend) called me to ask if I was coming, because she and T wanted me to meet one of their male friends. Sure, fine, whatever. I said sure, I'd come. This to me felt like an adequate word-of-mouth invitation since P is T's girlfriend.
R emailed me today to ask if I was riding down to the party with them. She also asked what I was bringing?
Yes, apparently it's customary to bring something. First I've heard of it. The party is also themed...this year it's a paisley themed. Again, first I've heard of it. I remarked that I was kind of irritated by all this. I STILL had no actual invitation, verbal or email or anything, from T himself and now I find that there are accepted customs to this party of which I am unaware. Nor has anyone bothered to email me with the time the party begins, or how to get to T's house.
R seemed irritated that I was irritated, saying that it's all just informal and no one gets official invitations, it's word-of-mouth, yadda yadda. Well, excuse me for living, but as a first-time attendee to this party, to have received NO acknowledgment from the host or information about the party at all is really rude. It's great that everyone else seems to be in on this big word-of-mouth loop but it's like everyone is assuming I know everything and in fact I know nothing. R also said that T had asked her for my email address, but to date I've received no messages from him.