Self effacing means modest, unassuming, not liking to draw attention etc. Is that what you're talking about? Okay, reading a book at a party is probably not a good way to blend in and not attract attention Neither is looking at your phone.
Best suggestions I can come up with are always look like you're talking to someone... blend in. If it gets to be too much, move on. Maybe entertain the little ones a bit. There's always a lot of trash and paper etc at a party... volunteer to take it out Ask people what's going on in their lives. People love that. It's nice if you at least pretend to listen to the answers lol. Could even be interesting.
Good luck... I understand