The intercom
Make a thread in the Saloon Bar, don't constantly bump it. Also, use flamboyant edit summaries that people can see in recent changes.
I must say, though, that hosting a game on RationalWiki is a rather infuriating use of server space. Not that there are any reports of server space shortage, but come on. You can instead get a free wiki on a wikifarm and have people join from here if you wish. That would be more palatable to the site's user base.
Of course, the general policy on user space here is that you can do whatever you want with it, but that doesn't mean people will like it. One person, Civic Cat, used his user space to host a listing of music videos with barefooted women (only after everybody decried its inclusion in fun space).