Force category watching based on group membership only once

Another useful modification of this plugin would be to apply it only to each user’s primary group. How would I do that? @techAPJ @alehandrof

Oh, and here is another idea that does not require any code changes, but would it work? What if I include various private categories in default categories watching first post, will that entail that whenever a user first acquires access to that category (by joining a specific group) s/he will be made to watch that category?