For future reference, this is what you have to do now:
Change "Configuring privileges for..." from "Global" to each category. You can then remove all "Guest" privileges.
Users can create a private discussion. Such discussions will be visible only to the creator. If the user wants to invite discussion of other people, then indicate - @nick @nick2 @nick3... (in the topic).
Discussion displayed will be in the selected category. Everything is as usual. But only for invited users.
It a similar idea.
What do you think?
It's probably doable via plugin.
This would probably be a good place to start, you'd basically just need to add the invite mechanic to this plugin: