There is a participant counter situated at the bottom right of the screen which keeps you informed as to how many users - including the session creator - are currently participating in the session.

 
Next to the participant counter is the Buddy List/Chat icon. When you click on that icon, the Buddy list / Chat window will open informing you of which users are currently joined in the session and their drawing colour. It also gives an OK if the user has located the current media file loaded in the main viewing window - or a message letting you know who is missing the current media file.

Chat is entered at the bottom and the enter key will post it to all other users in the session at that time.

User will only get see the Chat record from the point they join the session. If a chat has been posted by a participant in the session and you do not have your buddy list open you'll see the Buddy List/Chat icon start flashing on the main screen. 

A history of the chat is stored in the middle box and is saved out when you save the session. 

 
You may change your screen name by clicking the 'Change My Name' button.