@petros My personal feeling is that these things should be thread-based - that whoever posts the original entry should kind-of own that space (although maybe it would be better to have clearer tools to separate sub-threads than just dropping the original owner).

Given that, I'd default to "followers of original poster".

Private block and follower lists complicate things. I think it would be possible with a layer of indirection, but messy. Much of this is effectively on an honour code anyway!