I’ve just released discourse-bcc, a plugin that allows you to send a individual personal messages to several users at once.
When enabled, you’ll see a checkbox when you start composing a PM:
If you check that box, instead of one PM that everyone shares access to, Discourse will create 1 PM per user provided.
If you add a group, each member of the group will be sent an individual PM.
The plugin is fairly new and while I’ve confirmed it’s working it should be considered a beta right now. Please let me know if you have any feedback!