To chat with other players an input field at the bottom of the message dialog is used. This control is hidden until a network connection has been established. You enter the text of your message in this input and send it per Enter to the receiver which can be chosen from the left hand drop down.
The menu of receivers lists all logged-in players, and additionally server, group, and kibitzes. If you select server your message will be sent to all players that are connected. With group all players in the current group will receive the message, including kibitzes. If you want to talk to only one player you should select his or her alias from the menu. And last but not least you can communicate with the kibitzes of the current group only. Messages are 'shout' to the server, 'said' to the group, 'whispered' to particular players, and 'tattled' to kibitzes.
Alternatively, messages to a single player can be sent per context menu. Right click the receiver at the tab game server and choose 'message'. A dialog is shown where you can enter the message text. The successful communication with one player is confirmed by 'You whispered to...'. In all other conditions you will receive the message yourself too.
Messages can be sent to players that are not online as well. You have to enter the command directly in the message input. Commands start with a slash /, followed by the operation (whisper), and the specification (the name of the receiver; in case of spaces you have to quote the string). All following text is treated as the message. For instance, "/whisper Scotty Let's have a play this evening!". If he has provided an email address a simple email is sent that a message is available. The next time "Scotty" logs in he will receive this message with the information who and when sent it. Names are case sensitive, that means "/whisper scotty Hello World!" will fail and you get the error message 'unknown receiver'.