This looks like crucial advice to me. Thanks a lot. By implication, you are saying that once a user has signed up, there is no way of subscribing them to a new category? For the (very realistic) use case you mention (sending out announcements etc) I wonder whether it would not be more advisable to add that category to the “default categories watching first post” setting, rather than “default categories watching”. It took me a while to figure out the meaning of that setting but I think it means that users will be notified about my new announcement but not about any comments to that post.