Update from January 2020 Post: No Better Way to Collaborate Other than Chat
Microsoft is eliminating the ability to initiate a chat directly with a co-author of Word, Excel, and PowerPoint document. This change will take affect mid-April (across the web, Windows, and Mac versions of Microsoft Office).
Moving forward, users can communicate with others working in Office documents using document Comments (on the Review tab on the ribbon) when you want to attach a comment to a specific area within a document. Use Microsoft Teams Chat when you want to communicate with others immediately.
When this change is implemented, users that are in the document at the same time, will still continue to see names of co-authors at the top right side of the document window, but the ability to initiate a chat with one of the co-authors from this location will no longer be available.
Click the link for more information about chat, teams, channels, & apps in Microsoft Teams.