You need to go to the "User Control Panel" from any forum page, top, and set up your signature, under the "Profile" tab, like this.
??????? = Your 7 or 8 Digit Player ID Number From Your Profile Page. NOT the one at the bottom of the game page.
This makes it easier for people to add you.
This is for portal players only, not Facebook. It's slightly different. Maybe someone will post the instructions for it sooner or later.