The Users section of the Retrieva preferences window is split into three vertical sections. The right most section is concerned with connected users.

If you are logged in as the Retrieva administrator, you can send messages to users that are currently connected to the Retrieva system.

To do this, select the user you wish to send a message to in the Connected Users List and click the Message User button. You will be asked to enter the message and then the message will be sent to the user.