Right now, users can follow "normal" forums for new topics. However, they can't follow "category" forums -- i.e. follow all posts in the category's subforums.
We'd like an easy way to do that. Maybe we could add a Follow button to the Category.
We'd probably expect to manage these on the Manage Followed Content page.
About Categories
In our Invision Community, we have a lot of smaller forums that are categorized via a parent forum configured with Type = Category, as shown below. Unlike "Discussions" and "Questions" forum types, there's no way to Follow a Category.