We’ve heard from many application developers that their pre-existing user communities include people who aren’t yet on Facebook. We’ve created special registration links which give your non-Facebook users a streamlined way to join Facebook with your application already added to their accounts.
You can use these special registration links to promote your application more effectively. The links point to a user registration page and orientation process that is customized for your application. To learn more and see screenshots of this workflow, take a look at http://wiki.developers.facebook.com/index.php/RegistrationLinks.
Of course, if you give a special registration link to people who have already joined Facebook, they’ll be redirected to the “add” page for your application, or to the canvas page if they’ve already added your application.