It's obviously easy to "add" someone to a thread by using an @ mention (for example, I just added @Bill_S to this thread).
However, on private messages there is an "add user" option and on threads there is an "Invite" option where you can put an email address. If you try to put a user name in the invite it tells you to enter an email address. If you put an email address of an existing user, it fails saying they already exist.... Could this be updated to have similar behavior to an @ mention? To a new user it might seem like there is no way to add an existing user to a thread, especially if you don't know about @ mentions as they are not a common feature on most non-wordpress forums.