While you're co-authoring a file it's often helpful to be able to have a text chat with your collaborators. In Office 2016 for Windows you can open a chat window that lets you easily communicate with everybody else who is currently editing the file.
Before we get started with this feature there are a couple of things you should know.
In-app chat only works if you have shared the file with others and they have the document open.
In-app chat requires the file to be stored on either SharePoint or OneDrive for Business. OneDrive Consumer, or any other storage location, is not currently supported.
In order to chat you (and the people you're chatting with) need to be using the latest version of Office 2016 for Windows, or Office Online. Office on Android, iOS or Mac is not currently supported.
To get started, look for the chat icon on the coauthoring gallery near the top right of the screen.
Click the chat icon to launch the chat box. Once it's open the chat is simple to use. Just type your messages and you'll see the messages others send as well. The chat history is not preserved, so if you close the document and reopen it later you'll be starting fresh with an empty chat. If you need more persistent discussion consider using the comments feature instead.
If another person opens the file to edit it, they can join the chat too but they won't see any history of the chat from before they opened the file. They'll only see new messages as of the time they opened it.
If you choose to minimize the chat you won't miss any messages. The chat bar will indicate if new messages have been received while the chat window was minimized. If you'd prefer to mute the chat window entirely just click the x at the top right corner of the chat window to "mute" it. If you reopen the chat window later in the session (by clicking the chat icon on the collaboration gallery) you'll see all the messages you missed in this session while you were muted.
Here's a very short video to show you how it's done.