_"...but presumably as an existing ID, not a new one?" _ I'm not sure what you mean here :) In the ASC UI I only see one "+" icon to add users and afaiu it is supposed to work for addresses associated with both existing Apple IDs and those which are not associated yet. Anyway, I also add existing Apple IDs to different organizations and it usually works fine. But this time we had to remove it from one organization for it to work with another. It might have also been something else that happened in the meantime, who knows. I think the same message will appear if you try to add an address that already exists in the current organization (which should of course result in an error, but with a completely different message). Anyway, something is wrong here :)