You can talk to all the players who are in the community (except during the 7-day trial period), unless they have indicated in their gaming preferences that they do not want to talk. So you can "chat" live on the game interface or send messages by e-mail via the website.
Chatting in the game interface
The chat box is placed to the right side of the chess board. After selecting the player with whom you want to talk from the list of players, you can type a message and hit the "Enter" key.
When your friends are online, they are automatically added to the list of players that you can talk to. Also your opponent will be added if you are playing a game.
In this list of players, when you observe a game, you'll notice that you can talk to them at "@game." This is a shortcut to send a message to all the observers of the game.
Sending a message via the website