Continuing the discussion from Send a Personal Message to an Email Address:
Is there a way to extend this so we can “add a non-registered person” to a PM thread by including their email address somewhere? I see in the UI that I can enter an email address using the Add or Remove … button in the block of existing particpants. But if I enter an email address there, the UI/UX looks more like it’s sending an “invitation” as I would invite an email address to an existing topic.
OTOH, the UX for starting a new PM to an email user seems pretty obvious. (Instead of typing a username in the composer recipient box, just use an email addreess and a little 
appears.) I’m just not sure how to loop these folks in to existing conversations.