Forcing people to do things when you’re paying them to do it isn’t particularly tricky, the answer is usually to make it as easy as possible.
I’m aware of a number of sites who have used a fork of discourse-watch-category-mcneel and numerous others who are falling over themselves for mandatory category subscriptions.
There are lots of professional settings where mandatory circulation is a requirement. Up until now that was achieved by distribution lists, but with the goal of getting ‘out of the inbox’ the ability to tie group membership to watched categories becomes vital.
In any regulated field, staff need to be kept informed. The responsibility for disseminating that information falls on a few hands within the organisation. Relying on individuals to manually take the extra step to watch a category isn’t an acceptable risk to them - they’re the kinds of situation where noncompliance will impact the organisation, rather than the individual. The problem becomes compounded when someone moves laterally, they’re going to need to change groups and receive a different subset of the organisation’s policy.