What you want is for members of a group to be forced to watch a category. If you are “watching” a category then you get every message posted to it.
See Make members of a group watch a category by default.
I think that someone made a plugin that will do this, but that it has no admin interface (that is, the group to category mapping is hard-coded in the plugin). You’d need to fork or copy the plugin and add your own group mappings.