You can get this level of control with group Personal Messages. For example:
In the first two messages, a member of the ‘writers’ group has invited the ‘publishers’ group to the message. In the last message, the publishers group has not been invited.
Only group members (and site admins) can access group messages unless other users or groups are explicitly invited to the message.
Admins, and moderators (if mods are members of the group,) can convert group messages to regular topics. So this might be a good approach for writers who are not quite ready to share their work. They could even limit who can access the message to a small group of users by creating a PM and inviting individual users to it.
A possible downside of this approach is that any user who has access to a message can invite any groups or users to the message. Ideally, there’d be a way to limit this ability to the message’s creator:
There’s actually a simple workaround for that issue by adding some CSS to your site’s theme:
.topic-owner.regular .add-remove-participant-btn {
display: none;
}
.topic-owner.current-user-post.regular .add-remove-participant-btn,
.staff .topic-owner.regular .add-remove-participant-btn {
display: block;
}
Unfortunately, that rule would be applied to all PMs on your site, so that only the user who created a PM and staff members would see a PM’s “Add or Remove” button. Also, the CSS approach to limiting invites isn’t bullet proof. If a user knew how it was being applied, for example if they read this post, they’d be able to circumvent it.
It would be great if Discourse provided a setting for this, either applied at the individual PM or group PM level.