I wasn’t sure which tense you wanted for the second part (past or present). If present, it would be, “But now that I’m in my postgraduate, I seldom hang out with my roommates. They prefer to stay in the dormitory,” or more casually, “the dorms.”

In American English, we rarely say “seldom”. We use “rarely” instead. It sounds a bit weird to say seldom. When I was studying at a university = when I was in college (this second version is more common and shorter)