Thanks for the thoughtful responses. I asked because I was simply curious if it was strictly undergrad or not. I know in reality I probably wouldn't try to join as a graduate student, it was just a thought that crossed my mind.
I am a brother of Alpha Phi Omega, which I joined my junior year of college. I just graduated this year, am 21 years old. I transferred schools after my freshmen year, which was a school that was 90+% Greek, and recruitment was during the semester, to a big public school where recruitment wasn't advertised to transfer students and was held before school started. By my junior year, it seemed to be too late. I see the sisterhood of the sororities and all the great things that come from Greek life, and always told myself that if I had the chance to go back in time that I would have rushed and hopefully joined a sorority.
Again, I know it's not realistic to try and join a sorority as a graduate student, I think that Greek life is great and it would be awesome if I could, but I know it's not realistic.