I moved your post to this topic. I think the last query in my previous post in this topic will get you close to what you are looking for. If your custom field is a confirmation type, enter ‘true’ for the field_value parameter. That will return all users who have checked the confirmation box for the field name you supply for the query’s field_name parameter.
The query will need to be added to so that you can filter out users who are already in your Pro group.