Look at the bottom right of the screen and there will be two grey boxes.
Press the first one marked ‘New Chat’ to say something to everyone who’s looking at the site. Everyone will see the message as a pop-up.
Press the second one marked ‘… persons here’ to select a logged-on user to send a private instant message to. Only that person will see the message, and may respond in real time.
Press black bar to minimise any unwanted boxes; or click the ‘cog’ symbol and then the ‘power’ button to remove the boxes from the screen altogether.
This feature was doing weird things so I’ve removed it temporarily – Martin.