Have someone in your sorority invest in the book "I Heart Recruitment":
http://www.amazon.com/Heart-Recruitm...1552458&sr=8-1
You're totally lost on how to COB and get new members, and that's understandable. That book is an incredible resource for chapters in your situation - where informal recruitment will make a huge difference, and the traditional methods of just hosting an open house event aren't bringing in the women.
Good Luck!
ETA: DGTess beat me to it! We posted at the same time, with the same thought!