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One thing that I (Alex) did for this (Spring 2012) class was, during the fall and until the class started, when I ran into somebody I knew who seemed possibly interested into Squares, I would encourage them to try it out, and then write down their name. Before the Fun Night and the class I emailed the people I'd written down a couple of times encouraging them to come, mentioning they'd expressed interest before and when. This seemed to work pretty well at getting people to come. (Somewhat key to this, I think, was actually keeping track of people who seemed interested and sending them personalized emails, including ~"I talked to you a couple months ago, and you said you might be interested in Squares. On Tuesday we'll be []...")
We might want to systematize this by, e.g., trying to poke most of the active MIT students every couple months, and encouraging them to keep track of people who have seemed interested. When we have an event coming up, poke them again to email the people they've recorded. (This, uh, may run afoul of the "we don't have many active students" problem, though maybe this class will alleviate that issue a bit.)
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