If you have shared a Google document with colleagues, then open the document, you will be able to see at a glance if anyone else is working on the document at the same time. You can chat with collaborators using the pane on the right-hand side of the screen. 

If a colleague that has editing access to the document opens the document, you will see that another user is viewing the document in the top-right corner of the screen. Position the mouse over their image to see their name. 

You will be able to see any changes that the other person makes to your document immediately, and will also be able to see the exact part of the document they are working on. 

You can chat with all collaborators currently working on the document.

If you receive a chat message in a document while the chat window is closed, a red indicator will show on the chat icon:

Note that the chat is not saved in the document – if the document is closed, the chat will be lost.