a sorority is like a club thing. so, sororities (for girls) and fraternities (for guys) relate to "greek life", and they usually do fundraisers and other activities. the period where you apply is called a "rush". you pay dues and have to dress nice and do activities. upperclassmen would be "bigs" and underclassmen would be "littles" and every big has a little to be a mentor to. idk, i don't really like them nor care, but i hope that helps! also, google, youtube & wikipedia might be helpful too!
Aaah okay, I don't know what upper and underclass men are though, we don't use words like that for years. I think we have similar stuff with our clubs you can join at uni here though, cause they go on trips together and compete (if it's that kinds, dance or whatever) so yeah