Hi, 

If you tag someone while logging a note in the chatter (@whoever) they will be notified. If you use "Send message" all followers will be notified. 

For Discuss, do you mean all your specific teams in various discuss channels? If so, yes, easily, you can add people in specific channels and set them to private. You can also have an Authorized Group which can be for example, the users you allow access to Sales. They will auto subscribe to that channel.

I hope this answer your question!