Well, if I could code as well as I ask for things… yeah I’d be all over it.
There are these requests
Make members of a group watch a category by default
Watching a Category, perhaps the best kind of mailing list mode?
But there’s nothing built-in yet that’ll do this. I’m about to have to take a dive into the database of one of my self-hosted discourse to manually add a new category to all existing subscribers, then tell them how to opt-out if they just aren’t interested.
The only thing I can imagine, at this time, to satisfy the OP would be to add a trigger in the database that re-added a category to a user’s watch list, if they are a member of a group, thereby ensuring they can’t opt-out.
Can’t do that on my paid account at this time, however, so I’m glad I don’t have the need there.