When using the User Registration Form (https://craftcms.com/docs/templating/user-registration-form), how do I make it so the user is added to a predefined User Group?
See the docs here:
Craft Pro has the option of allowing public user registration. It is not enabled by default, though. To enable it, you must go to Settings → Users → Settings, and check the “Allow public registration?” setting. With that checked, you will also be given the ability to choose a default user group that publicly-registered users are assigned to.