I may have misspoke because WP does not support taxonomies on users. That option would really not be viable.
The best option I can think of is to use user roles
What you would really need to do is to create a select box with the groups. Then you’d have to use an acf/save_post action to get the value selected and set the user’s role based on the values saved in the field.
In your action you’d want to make sure a user is being created or saved, the $post_id value in this case would start with “user”, (ie: "user_{$user_id}").
