Is it possible to invite a user to the portal by URL?

Is it possible to invite a user to the portal by URL?

Just a scenario here: 
I would like to invite a user (the client) in a portal via a URL. For some reason, I might not have their e-mail address, or perhaps they want to associate this account with a different e-mail address. Is there a way to generate a one-use URL for signing up to the project, which would then assign them to their appropriate role?

Thanks