An Office 365 group makes sharing files easy by including a dedicated library, which is the primary place for storing all your group files.
When you create a file or folder in the group's library, all members of the group will be able to access it. When you add new members to the group, including guests (external users), they too will be able to access the files.
Post files to the group library
To share a file on your computer with a group, just drag it from your computer to the group's library. See Upload files to a library for more information. If the file you want to share is already in your document library, you can keep it where it is and invite the group to it.
Note: To delete or rename a file, first navigate to the SharePoint document library by clicking Browse library towards the top right corner. Right-click the file you want to delete or rename and choose Delete or Rename as appropriate.
Share group files with guests
You can share group files with guests by attaching any file stored in your group library. See Guest access in Office 365 Groups for information about guest access in Office 365 Groups.Attach a shared file
From your group, start a conversation.
At the bottom of the conversation pane, select .
Select the source of your file: group or computer.
If you select a group as your source, select the file you want to attach.
If you select Computer as the source, select either Upload to group files or Attach as a copy. Uploading to group files enables group members to see the latest changes and co-author in real time. Attaching as a copy gives each recipient their own copy of the file for review.
Note: When a group includes guests, you might see a message like this one informing you that attachments to guests are blocked. Your guest will not be able to open the shared files.