Definitely talk to the greek life office, and of course, keep an eye out for advertised COB events.
Also, talk to your friends who are in sororities, and let them know you're interested. If you don't know anyone in a given chapter, get to know a couple of members. A sorority that can only extend a few bids is not going to advertise their COB events; the sisters are going to think about women whom they already know, and invite just those women to their COB events.
Good luck.
