Cream has some good ideas. If I were you, I wouldn't go crazy trying to hold a "RUSH" as it were. Pull your core group together, and have an informational meeting or two - you might find a few more women interested in becoming founding sisters of a new sorority. At this point I wouldn't worry about planning rush parties and such.
As your group grows, you may want to think about electing officers and putting a procedure in place for inviting women to join your group (rather than taking all comers). Your group will be, in essence, a local sorority.
Don't worry about people not wanting to join your group. There will always be interested women out there, who either are not interested in or don't get selected by the 2 other sororities at your school.
Lots of luck, and Happy Holidays
