I’m not sure if this is the most recent or relevant thread on the topic, but when sending a direct message to somebody on another message board, I saw that there was a checkbox (unchecked by default) which read “allow other members to be invited to the discussion”, or something like that. I think that would be great to have for Discourse: it feels important to me that when conversing in a “private” (closed) space, participants have control over who can be added.
cf. Whatsapp groups, where new people who are added to NOT see previous message history, versus Messenger discussion groups, where they do, which can create no end of problems when sensitive information was shared amongst a small group (intended audience) and suddenly is revealed to a wider audience, without consent.