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:
Go to the group you want to delete and click Members on the left
Click ☼ next to each member's name and select Remove from Group
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.
Get help from other people on Facebook with similar questions, visit Facebook Help Center.
Tip: So, you made a Facebook group but there’s hardly anyone in it? Here’s some of the best tips on how you can make your Facebook group popular from Makeuseof.com.