There's an effort at Dartmouth to see if a seventh sorority could be established. (Right now, there are three NPC groups -- Alpha Xi Delta, Delta Delta Delta, and Kappa Kappa Gamma -- and three locals.)
The student paper's article (link below) gives several reasons why some want a seventh sorority: to give women more options, to reduce the size of chapters so bonds and friendships are closer, etc. Possible processes and possible barriers are also outlined.
http://www.thedartmouth.com/article....=2005041901010