First years hang out together- this tends to be obvious due to orientation, classes, TAing, and the lack of a lab to live in. Sometimes this group splinters into Chemistry disciplines (Organic, Inorganic, Pchem), although I assume this depends on how big the department is (ours is rather large).
By about the middle of 2nd year people splinter off into hanging out with their groups- generally you rarely see anyone outside of lab, so this is natural, as long as you actually like your group. There are also groups who tend to hang out with specific other groups depending on the situation.
Ok so those are the basic rules. I happen to meld a few of my first year friends into my hang out circle, not sure how common that is. It also helps that I live with one of them... plus I'll meet the 1st years due to my 1st year roommate.
So at this social this is what normally happens. People show up with their groups, often they will see some long lost year-mate and wander over to talk to them. Creating either little clusters of lab groups or little clusters of year-mates, generally there isn't much mixing besides that. I feel that this is really not optimal to our socializing, but have yet to figure out a way to fix it. So I just try to actually talk to people not in my year or my lab group.
Anyhow, that's my morning thoughts... time to get some lab work done before this lovely social (they do give us free food, which is nice even if it is pizza).