I wanted to know what would be the most efficient way to edit join.php to allow those registering to choose their own permission group, or to set the permission level at time of registration for users.
How sure are you that you won't have problems with users selecting the wrong groups? Will one of the groups have higher permissions that you need to be concerned about?
If so: I suggest doing join keys where you create a code linked to a permission level that allows signup to that, and add it to the additional_join_form_fields and after_user_creation.
If not: Add a dropdown/checkboxes similar to the ones on admin_user or admin_page.php for permission levels to the join form, and run the PHP in the after_user_creation.
Brandin.