I'm not anti-social by any means, but I do think it's rude to invite yourself to sit at someone else's table. I would never do that and I would feel a bit weird if someone else did that to me. However, I also think it's rude to sit at a large table if you are only a small party. We've never run into this situation because we always have at least 4 people. I don't mind sharing at Beirgarten because it is expected and everyone knows going in that shared tables are the norm.