If you need to revert to draft (e.g. if the group has submitted the wrong file), you will need to do this for all members of the group. Likewise if you are granting an extension to a group you will need to grant the extension to all group members.
Steps to manage group submissions
1. Use the Separate groups filter at the top of the assignment submission screen to show only members of a particular group.
2. Click the select all checkbox at the top of the table to select all students in that group
3. Select the action you wish to perform, eg. Revert to draft or Grant extension from the With selected menu at the bottom of the screen.