By default, new users sign up using the built-in sign up pages, at app/sign_up.jsp.
As an alternative, a custom sign up page can be provided, using the Sign Up node type. This requires an account template reference, and provides options for controlling the sign up page, including whether to auto start it, whether email input should be on a separate page, additional content to show, and a template for the email that is sent.
Give the anonymous user execute authority on the sign up page, and direct users to the page to sign up. Link to the sign up page with a redirect parameter to redirect back to the linking page if the user cancels the sign up.
Account action with custom sign up
The Account action processing can be configured to use a custom sign up, by setting the Account action sign up node property on the theme to the reference of the sign up node.