From the developer who brought us twtpoll, Felipe Coimbra, now there is twtvite, an easy way to use Twitter or Facebook to create a micro-invite for your friends and followers. You create an invite on the twtvite site, and then micro-message with a tiny URL on Twitter or Facebook. The link takes people back to the site where they can RSVP. (See screenshot below).
Coimbra is creating a whole suite of Twitter apps, including twtvite, twtpoll, and twtcard. You can find them at TwtApps.