You can email them a link that takes them directly to the registration form. You would need to set up one first.
You can add a link for the user registration form to the portal's login page. Here's an example:
The password (or any login field) would be a column in your users' table. So you would make your own logins in Airtable. That information would be used to look up records and would not be provided by the extension itself. To allow each user to set a password, you’d add a new text field for passwords in your users' table. That way every user could log in using their own password.