SAM
Captain America Sam was, of course, helping with the food and was probably still getting it laid out when people started arriving so it was a pretty simple transition to "being at the party" from there. He's very much on the lookout for his double - he has questions he wants to ask.
Unfortunately for him, Winter Falcon Sam is not a party person. He doesn't even entirely like being too far inside a building; he gets antsy if there isn't an openable window he can easily escape through. Normally he'd stay far away from something like this, but the lure of food has him scoping things out from outside, using the windows to see what's going on. Anyone wandering around outside for some reason is more likely to spot him, he's mostly hiding from the people at the party itself.