The chat link in the navbar shows who is in chat, but if they leave the chat system sometimes the navbar count still shows them as in chat. You can refresh the navbar by right clicking on it, not on a link, and choosing 'This frame' and then 'Reload frame'. It will usually refresh the count and show if there is actually anyone in the chat system.

Once you're in a chat room, if there are others on the system but in another room, you can check the chat listing drop down and it will show you how many are in what rooms, but who they are, or you can use /whereis Username to find a particular user.

The little blue question mark in the lower right of the screen when you're in chat will give you a list of chat commands and a description of each.
 

It's not allowing me to choose "This Frame" or "Reload Frame". It only allows me to refresh with that little circle arrow symbol. Ah well, maybe if someone constantly remains in a chat, it might bring folks in. :D