Groups are deleted when they have no members. If you created a group, you can delete the group by removing all its members and then yourself. Admins can't delete a group they didn't create unless the original creator chooses to leave it.
To delete a group:
1. Go to the group you want to delete and click Members below the cover photo
2. Click next to each member's name and select Remove from Group
3. Select Leave Group next to your name once you've removed the other members
Note: Deleting a group is a permanent action that can't be reversed. If you're an admin, you can archive the group instead. Learn more about the differences between archiving and deleting a group.