Groups are automatically deleted when they are empty. If there are still people in the Group you cannot remove it manually, even if you created it. If you are the only person in a Group, it is deleted when you leave.
Single-person Groups that are not Premium are deleted after 48 hours; Groups are also pruned daily to remove inactive users from the Groups and ensure a good user experience.
Please note that Premium Groups will not delete when the last user leaves.
Please contact support if you need to delete your Group and it's Premium.