Yeah, them dadgum foreigners.
I think I went to at least a couple pre-interview dinners. Maybe 1 or 2 more post-interview dinners too? I recall interviewing with anywhere from 0 to maybe 3 others on my days, so it was never particularly busy. They told me up front whether an attending would be there, which I think only happened once. I wore non-black slacks, a button up, tie, but rolled-up sleeves. In all cases I loosened the tie pretty quick, as it became clear it was a casual dinner just to relax and get to know people. Unless they tell you you're going to paintball, something in the realm of smart-casual is what I would recommend.
I lean towards overdressing, then changing my composure to meet the setting. You can always kick back and relax in a suit, pull off the coat & tie, etc., but it's hard to put up a classy professional aura in shorts and sandals. As clothing goes, the only 2 times I remember people commenting negatively were about one chap who couldn't thread his belt (also couldn't hold a conversation -- great numbers, subzero social skillz) and another who showed up in jeans and tennis shoes or somesuch (but had some excuse about his suit -- despite this he ended up matching with the program and was joked about it mercilessly, so the story may have inflated with time; I think the last time I heard it, he supposedly was wearing a dirty wifebeater).