This outfit, Center for the Studies of the College Fraternity
http://www.indiana.edu/~sao/cscfsite/ published a pamphlet outlining achievements, famous people etc. of fraternities and sororities. It mentioned how most all-sorority GPAs are higher than all-women or all-students, graduation rates are higher, outstanding leaders who are Greek etc. Also check the NPC's Web site,
www.npcwomen.org, I believe they also have persuasive information. You might also ask your parents to talk to your school's Greek advisor.
You need to show your parents that you have
maturely thought this out, explored the idea and show them the reasons you think it wold be a good idea to join (and that it's not all parties). Good luck to you!