What my team did was had an info session/social lesson. If you have some sort of ballroom place on campus with hardwood floors, try seeing if you can reserve the space. What we then did the weekend before the event was we chalked on the sidewalk around campus (main spots being dining halls and the library) and made a facebook group/event. We also told all of our RA friends to tell their friends and residents to come to the social lesson (and if you don't have any RA friends, make some RA friends because they're awesome).
Unfortunately, it rained two days before the event, so we had to do the chalking the night before, but we still had about 60+ people show up for our info session/ social lesson (which is awesome for such short notice); have a few of your members do a sample dance, a couple at each level (if you have it). Then it can be like "this is what you will learn after x amount of time" after you have the dancers introduce themselves and say how long they've been dancing. And about half of them came to our competitive dues-paying team lessons. Hopefully more people will show up next week since there was a huuuuuuuuuge basketball game the night of our lesson (so make sure there aren't any huge scheduling conflicts. ;D). Advertising (handing out flyers, chalking, using the newspaper, networking, etc.) can be really effective if you use all forms of media that is available to you on your campus.
Phew. I wish you the best of luck.