Chat must be unlocked first which is simple. Just click the agree to EULA and accept the server policy on behavior which is basicaly keep it clean and nothing abusive.
To send a user a private message type tell username message. i.e. tell rookers hi. If someone has sent the user one the default color is yellow and says so and so tells you, there will be a send prefix with "tell name" available to select. Click/tap the Send Prefix button in console in lower left for this option. To stop sending that person messages after the send prefix is selected. go back and choose the send prefix for command.
To talk to opponent or last opponent, type say message. i.e. say good move. The Board's Actions button will have chat options as well that will be based on the type of game. When playing tell opponent is a choice, when observing it's whisper, and when examiming it's kibitz.
If the settings "says in Main" is selected on settings tab, says will go to both game console and Main View. So a converation with an opponent or last opponent can be had in main view console tying say blah blah to talk.