I feel @Stephen gave you the solution without really knowing it: Private messages.
The person creating the PM can add a lot of users and in the end, it pretty much works like a thread. Try it, you’ll see. As for being part of a “category”, if the thread is private, thus not visible publicly, and only accessible by a specific list of individuals, there really isn’t any need for a category. Just create one to host the public threads, and explain to your pros/coaches/instructors to create a private message to their first contact (or themselves if they want to start without anyone), and then they’ll just have to add additional users.
Just try it by yourself.