Hi, I’m just a girl who was recently accepted to a graduate school program at USC for 2 years. I’m coming from Southern California as well, but I didn’t do my undergrad in Los Angeles. Thing is, I didn’t make any long lasting friendships while I was at college, so I don’t have many friends. I was hoping that attending grad school in the same area I grew up in could not only let me have job opportunities closer to home, but make new friends in the area too. I’m just worried that the other students won’t want to make new friends since most of them are already in their “cliques”. I know everyone says to get involved, but are there other things I can do to meet people? Thanks!
Making friends is not a discrete event. It’s a process, it takes time, it takes effort, ongoing effort, it takes authentic connection, genuine interest, caring even if inconvenient at times. It can happen anytime, anywhere, at any point in life. Of course you can make friends in graduate school. You can make friends anytime in life. But it won’t happen if you hide in your apartment. Be friendly, be open, put your cell phone in your purse and actually converse. Show interest, invite someone you want to be friends with to do something. Keep at it. Other thing is that making friends can happen at the most unexpected times.
What kind of grad program did you get into? I know a lot of them can be quite small. I did my undergrad at USC and it seemed like absolutely every student in the linguistics department knew each other. Granted, the undergrad program has about 60-70 students according to my advisor, so that might have been why. As is suggested above, you need to try reaching out more if you haven’t already 
The grad students in the linguistics department regularly had Christmas parties with ugly sweater contests, Halloween parties, etc. If your department has such events, make your best effort to attend them. I’m a bit reserved myself, so I should follow this advice as well if I get into any grad programs. My circle of friends at USC was quite small. I do get what you mean about cliques though. My major had me taking classes in Dornsife and Viterbi, but I never really made friends with any Viterbi students.
Try asking in the graduate school subforum: http://talk.qa.collegeconfidential.com/graduate-school/
or at Grad School Life at https://www.chronicle.com/forums/index.php?board=42.0 or
or http://talk.qa.collegeconfidential.com/university-southern-california/