I would like this feature too. I sent a lot of invitations for my community to sign up a new account so they have to go the web site, click Sign Up, choose a username, enter the email, a password and the invitation code. With the exception of a user name and password, it would be nice to be able to create a url that does all that so the Sign Up form would be displayed directly with the most of fields already filled. It would make the process easier.