PM Forwarding is now live. You can now add new people to a PM thread where you see fit.

To do so, simply click to view a PM thread. At the bottom of the thread is the reply form. In the recipients field above the subject field, simply click the field and enter the name of the user you wish to add to the thread. Then click submit.

The new user will be added to the thread and will have full access to the entire thread as well as the ability to reply.