Assign registering user to multiple group based on checkboxes choices

Hi, I'm using a CF6 form to manage users registration on my website.
I'm in the need to assign users to one (or more) user group selected during registration via checkboxes.
I've tried to follow this FAQ (https://www.chronoengine.com/faqs/61-chronoforms/cfv4/cfv4-tutorials/4618-how-can-i-add-a-user-to-a-user-group) but i can't do it.
Can anyone help me?
Thank you.
ET

You should be able to use the Joomla user action, there should be a user groups section there that will take an aarray. Use a PHP block to return an array and then use {var:phpBlockName} in the user group ID section. You can use form data directly with {data:checkboxrs_group_name} but I wouldn't do that dire rly that's a terrible idea