In the lobby you can type messages to other clients in the lower left corner.
The client-server can select the next map to play by clicking on the small button next to the name of the current map (in the top right corner). Once a map has been chosen, the server can start the game by pressing Go!.
You can click on the small icons which represent each connected client, and some information about each client will be displayed. If it's necessary, the client-server can forcefully disconnect a client by pressing the Kick button.
Once in the game, the client-server can return to the lobby by pressing . A new map can then be selected, or the game ended.