At Admin Panel -> Settings -> Switches you have an option to activate a chat room. This is a refreshless, AJAX-based chat for members only (not unregistered guests, though they can watch).

Once chat is activated, you can set which usergroups can view the chat in your usergroup permissions (Admin -> Members -> Manage Usergroups).

A new chat settings page will appear at Admin Panel -> Settings -> Chat. Here you can set the order of messages, the size of the backlog and the data-fetching delay. Setting the fetching delay to a larger number will reduce server load, but increase chat latency.

There's also a "currently chatting" list of people in the chat below the online users list on the front page.