Instead of having it be completely random, I am thinking of a site similar to sub.rehab.

Instances that are open to public signup would register on the site, and give updated states on how many users they have along with a brief description of what they are for.

Users can decide roughly what size instance they'd like to join and view a list of matches. We could add a "I'm Feeling Lucky" button that picks a random one out from the list.