Add customer to Joomla User Group in registration form

Hi,

You can try to do this:
First, create a custom field of the table "user" and of the type "checkbox" with one value via the menu Display>Custom fields so that customers can get that checkbox on the user registration form.
Then, create a mass action on the trigger "after a user is created" with a filter on that custom field column being equal to the value you entered in the custom field, and an action to add a user group.
That way, the mass action will trigger automatically after the registration, check the value of the custom user field and add the group to the user if it corresponds to the value