Collaborate on files in Teams

Check out this guide from Microsoft: Collaborate on files in Microsoft Teams 

Items to note about Teams file collaboration:

• Co-authoring is ONLY compatible on .docx, .xlsx, and similar files. You will NOT be able to collaborate on older files, such as .doc, .ppt, etc. (the ones without the x at the end) 
• Your personal OneDrive file storage is accessible from within Teams, as is your shared Teams/SharePoint file storage. It can be easy to confuse them, so it's important to understand the differences. See Understanding the differences between Teams, SharePoint, and OneDrive for more information.
• Read more on Teams Guest User and Staff file collaboration here >>