This is an interesting area to work through ...
If you have the user's
tax id and you know the role they need, then why not pre-register them and then send them an invite to login?
From my experience with WordPress, it's easier to bulk add users and have them partial registered. You can import users with a plugin like this.
You may already have read .. there is a bit of a post on WordPress.org about having 2 registration forms ... one form will change the users role after registration.
If everyone who registers has to be an 'author' or have special privileges, then change the default role from Subscriber to Author (WordPress Admin > Settings )