@hoergen Hmmm, maybe I'm all off here, but ... this somehow feels like a problem that has been solved in the past, and be that as simple as in e-mail: Sending messages to a wide load of recipients with all of them in Cc (because I want to discuss issues with all of them), anyone who responds to any of these messages will send this response to all of the original recipients, and that is intentionally and expected to be this way. If someone modifies the set of recipients - fine, of course everyone's able and allowed to do that, that's a conscious decision, but it's not, like "no matter what - your response will only be seen by those people in that recipient list that have you in their address book"... . Maybe this analogy is a bit difficult, but at least that's a kind of behaviour that, for an addressing as generic as "Following", would somehow be not all too much off.
@hoergen Hmmm, I don't see the problem here to be honest. I don't want to explicitely add people to a particular group or forum. I just actually would like to be able to send out posts avoiding they're visible to everyone globally, either by looking at some "explore" timeline or by knowing a link to a post. I'd like to make a restriction that limits posts to be available to people I follow (or maybe mutuals of mine) without enforcing that these people necessarily have to follow each other too to reach each others posts. The latter might be perfectly valid but it seems a different use case at least to me.
@hoergen Forum / group seems similar but unrelated; in this case, don't I need everyone I would like to address to be in that group / on that forum then?