[EricH]

You could always declare 2nd robots free (but subject to "the boot" if more 1-robot teams happen to show up). That should give more robots.

The "volunteer bounty" can work pretty well, too. (Discount for certain key volunteers)

Offer "fun" stuff that isn't necessarily competition. Say, obstacle course, HP competition, or similar items. (I bet the words "mentor" and "match" used together would get some pretty enthusiastic response... just a thought...)