login by a url is always possible... not secure at all but possible... for the registration, simple... if you have cookies for your actual login process, the process can verify the cookie and create the user and login...

that process would be simple... the cookie have the username and password, so a simple modified version of the registration process can call the cookie infos and insert in the proper fields in a form, the user have simply to click "register" to proceed...

BUT...

nothing can be secure in that process because you can't provide a secure cookie - even if you set it to be coded...