It may SEEM that the social groups are already formed, but they haven't! everyone is lying to themselves! and each other! social groups are SUPER flexible, ESPECIALLY this early in the year. and alcohol makes everyone super friendly anyways.
do people still pregame really openly in the hallways? if that's the case, you're in luck, because all you have to do is say "Hey, I live upstairs, this looks fun, whats up" and they will, four times out of five, say "WE'RE PLAYING SOME STUPID DRINKING GAME AND WE NEEDED ONE MORE PERSON THIS IS PERFECT, COME SIT DOWN."
I literally met my current boyfriend (who I have now been dating for five years and have known for almost eight) and many of my closest college friends by looking out my keeney window on a friday night, seeing a dumbass strobe light and stuff in a window across the quad, grabbing the somewhat-quiet girl who lived next door to me, and saying "looks like something's going on over there, let's go see." she was 100% not interested but i really wanted a buffer in case i embarrassed myself. sorry to put you through that, Alex from Florida!
as for the social anxiety: taking the initial risk will feel like getting shot in the stomach. it will be extremely scary the first time. but once you do the one scary thing, you're closer to doing what you want to do.
plus! people are super non-judgey. if you get overwhelmed in a situation you can just like... pretend to get a text, and dip out of there and go back to your room, and everyone will assume you're at a different party or something i guess. so no one will judge you for that, if you need to escape!