Ok I think I figured this thing out.
To add people from the Game Center (with Play IDs), you have to be playing the game through the mobile app. When the Message Center comes up, click the Community tab. You can change your own ID from the reassigned one, to something that you like. Then next to that you will see an ADD button. Type in the other person's Play ID and press the add button.
I have not been able to determine if they show up on your ribbon yet.
Friends added through FB, show up almost immediately though.