You can share files with your members in two ways. First, by uploading files to the Files section in each group. And second by attaching files to your messages.
To attach files to your messages, click the Upload File button:
Choose whether you want to attach files from your computer's drive or from within the group.
If you choose to attach files from the group, select the file(s) you want to attach and click Add.
Once your message is sent you'll be able to see the number of people who have downloaded the file and even take follow-up actions with them: