Well, if you have already submitted your move, and they haven't viewed the game yet, you can go back to it and click "Edit" near the top, and add a message. If they have already viewed the game (but not yet played) you cannot do that.
(hide) You can send a message to your friends with just one click by adding them to your friends list and then clicking the small envelope by their name. (pauloaguia) (show all tips)