Microsoft Office Tutorials: Co author office files

Try it!

When you save your Office files in OneDrive, SharePoint, or Teams, you can work on them with others—at the same time. You can do this whether you're in an online app or a desktop app.

Select Share in the top-right corner.
Enter the email addresses of the people you'd like to work on your file with, or if you have their contact info stored in your address book, enter their names.
Type a short message if you want.
Select Send.