This would require a plug-in specifically designed to work with the chat room in order for me to display who/how many members are currently chatting.
Please note that such pluggins can sometimes be clumsy, especially when integrated with vbulletin pages (like who is online or forum home who is online). The script can be slow or can get the server busy when integrated into high traffic pages (eg. vb forum home). We did it for real chat's pluggin and put "who is in chat" into vb forum home but it slowed the server down so we removed it. FYI.