There are two ways, the moral and immoral way:
Moral Method:
Send a message to kohai or submit a ticket explaining the situation. Someone should be able to help you out, and they will delete the group.
Immoral Method:
Create a new name for your group, advertising it to people. Send out some invites to premium members, and then leave and make him SA when one joins. (Just joking, don't really try this.)
If no moral methods of these work, maybe you'd be interested in having your own group? You can change the name and create all kinds of fun activities for your new group. Still no? If this doesn't work for you, see if anyone you know would be interested in owning their own group, and give it to them.

I joined a group that a friend of mine started about a year ago (with whom I've lost contact) . It only has one member -me - with my friend or former friend being an administrator. I don't recall this, but apparently I was made a super-administrator. I tried to resign from the group, but was told that I couldn't because every group needs at least one super-administer. Help!