These two plugins (GitHub - arpitjalan/discourse-watch-category-rzim · GitHub,
GitHub - amical/discourse-watch-category: Watches a category for all the users in a particular group · GitHub) do that, though they need to be customized by modifying the code rather than in system settings.
Let me know if you want me to do that; it’s probably an hour or two, depending on how many groups we need to configure.