Hi @Emily_Roman — not sure if this ticket really is about multi-homing.
Just to make sure: At least in my experience the issue has been about the lack of a dedicated home for messages to begin with, and increasingly so with agency clients/guest users in collaboration projects where you can’t do onboarding on the same level that you manage with your staff members.
What our clients try to do is stick to the rules and be transparent in communication. So they don’t send emails, they shoot us messages. 
However, intuitively messages get send to people, not projects. And most of the time, I assume, clients won’t compose these messages from within their project. They use the 
icon or that button on their inbox page. A user name or two get added and the message is sent privately, but they go about it without much consideration or intention. They wouldn’t even try to add a project name, although there’s your description naming the option, I know.
What clients end up with more and more often is producing messages cluttered all over the place, hidden somewhere in private reports with multiple sets of recipients. When clients realise (or get told) what’s happened there is no way of turning it all back. A project can’t be assigned after the fact, not even on their behalf, to keep things tidy.
Sometime I wish they’d have sent an email now. The old kind of cluttered information. Because then I could just forward it to the right place and get everybody up to speed with one action … currently I have to keep adding people as followers and post reference links back and forth so nothing gets lost.
Bottom line: It’s become more overhead not less, because I can’t move a message to a new home.
Or am I getting it all wrong anyway and there is a workaround to add a single project after the fact? Thanks in advance!
Cheers
Daniel