Quick question… if one or more users are tagged within a message, but are not invited to that message, do they get notified/see the message at all? My feeling is that they don’t and need to be invited to see the message at all.
Mentioning someone in a personal message when they do not have access to it, will (usually) produce a UX warning in the editor, because it won’t work. You can do it, but they won’t get notified or gain access to that message.