Sorry for the hijack. About to start a PM with someone the simplest way to do it is to click on their username in one of their posts (but make sure you're logged in). A brown drop-down menu appears. Select "Invite <username> to a private topic." This opens a reply window identical to the one used on Open Forum. Hope that helps.
PS I'm on my iPhone. On a regular computer it might take a right-click (click on the right-hand mouse button instead of the usual left) to make the menu appear.