lyly--
I think you would want to promote your organization as a growing one. Make sure women are aware of all of the opportunities in leadership that are available by joining your sorority. Many woman want to join a sorority and make a difference within that group, and your sorority provides that opportunity perfectly.
Inviting your alumnae is a great idea, and one which I think will work, because it shows that while you're small, you stick together and it's not just about membership in college but membership after college as well. Focus on that as well
One note: Morale has to be your number one priority. If you all go into recruitment with the "we're the smallest on campus" and "we can't compete with big sororities" you are doomed to fail. You have to stay psyched about your chapter, and most of all, motivate your members to be as psyched as you are. PNMs can sense lack of confidence in chapters so easily, and it's something that will drive them away.
I wish you the best