check out your local chapter of the
junior league! some chapters accept girls in their early twenties and while it's probably not going to provide you with a traditional collegiate sorority experience, the junior league offers tons of opprtunities to make new friends and better your community. it's worth a shot!
edited to add: also, you might look into other sorority-esque service organizations at your school. texas tech, for example has two -
miller girls and
fashion board. fashion board especially was WILDLY popular while i was at tech - they had mixers with fraternities, their own presentations, crush parties, formals, etc...