Continuing the discussion from Invite user to a topic inside a restricted category:
Pretty much in the quote. Allowing a user to @mention someone, or similar, and give that person access to read and reply only to that topic.
My society is moving towards discourse for both general discussion between members, but also discussion between members of the exec (executive board).
Most discussions for the exec are private, and thus people not on the exec should not be able to read them. However occasionally someone external to the exec can contribute to a discussion (e.g. bring their skill set to the table).
Instead of adding this individual to the exec group (and thus granting access to all discussion), @mentioning them in the relevant topic would provide them immediate access to past topic discussion, and allow them to reply.
As far as I can tell, at the moment the only way add a user is to topic (in a private category) is to add them to the group, or for the discussion to happen in a private topic. This is inadequate because it either requires giving them access to everything (via the group) or having the foresight to start the discussion in a private topic.