Here are a few thoughts.
Regarding sisters inviting friends, etc. Perhaps you could divide the chapter up into groups (like your colors -- so for DG, it'd be the Bronze Group, Pink Group, and Blue Group). Have a weekly contest to see which group can recruit the most women -- have some sort of desirable prize for the winning group.
Make sure you have some good PR t-shirts promoting sisterhood & friendship. Require sisters to wear them every *insert day or days of the week here*.
Serve hot chocolate outside popular campus buildings -- perhaps tie a ribbon around the cup with a paper attached giving information about your philanthropy --- AND a note saying if this interests them, that your chapter is currently recruiting.
Publish ads in your school newspaper with chapter news -- "Congrats to Susie Q, Donna Doolittler, and Cindy Lou Hoo for making 4.0's this semester!" -- and add a line about - interested in XYZ? Contact
lolo@lolo.com
Chalk the campus.
Do some community service work with the local HS (doesn't help with COB now, but will help in the future) -- try to contact the local HS Key Club or similar.
Event ideas:
- Game night
- Go to a sporting event together (sometimes something smaller than a football game could be good -- like a gymnastics meet or something -- ESPECIALLY if you have a member on the team).
- Go to a pick-your-own fruit farm.
- Laser Tag or Mini Golf
- Any philanthropy event.
Girls that COB are more likely to be a little skeptical about Greek Life (perhaps why they didn't do formal recruitment) - you need to find ways to show them what your organization can offer them -- sisterhood, scholarship, and philanthropic opportunities.
Good luck!