Good question!
My campus does formal Fall Rush, so we usually try to meet freshmen before school actually starts -- however, I think some of our activities might work for you also.
During freshman orientation, we always have a booth set up in the student life center to give out information about the sororities and fraternities on campus. It's been done two different ways:
Some years, all of the Greeks have one big booth and we have representatives from each GLO in their letters working the booth at scheduled times to answer questions of parents and freshmen about the requirements associated with Greek Life.
Other years, each GLO has their own booth set up so that the freshmen and their parents can get detailed info about each group instead of just a general idea about Greek Life on campus.
At the booths, interested freshmen can sign up for a mailing list, so that the Greek Affairs office can send them a Rush packet.
Another thing that I have heard of is for the Greeks to sponsor a Welcome to Campus barbeque on front campus, so that the freshmen will see it on their way to class. They stop for a hot dog or burger and meet some GLO members in a casual, no-pressure environment.
Hope these help!
Good luck!!
