I believe that Phi Sigma Sigma is the only group that will allow graduate students as new members.
They do not have a chapter at MSU.
However, contact the Greek Life office to inquire if any groups are in need of advisors - if your background qualifies you, this might be a great way for you to get involved.
Also, do a search on the topic in Rush - there have been many discussions on it.
http://www.greekchat.com/gcforums/sh...duate+sorority
http://www.greekchat.com/gcforums/sh...duate+sorority