You can find the chat in the bottom-right of the screen.
At the bottom of the side-bar, you can see the chat input area, and where to send it.
If you click on the "To" option, you can see a list of people available to chat, you can select an single person if you want to just chat with them.
Another way is to look at the list of people currently logged in.
This will again show a side-bar with people available.
You can click on a person, which brings up a window, and select "Message" to directly message them
