Discussions in Aptter have two kinds of “people” who can interact.
A participant is someone who has at least once posted in a public discussion or to a private discussion after invitation.
A participant is thereby always a follower so he gets notified when something happens in this discussion.
You can add people just as followers if you think that this particular discussion is one they should know about and get notifications.
Everybody has the right to “unfollow” or “follow” a discussion as long he has access to it.