To delete a group first you need to find the group on the groups page. Then click the "Options" button in the last column of the table and choose "View Details".

From here click the "Options" button and choose "Delete Group":

This option will be greyed out and it won't be possible to click if there are people in the group. It's only possible to delete a group if there are no people in it. If there are people in the group then you'll need to remove them from the group and add them to another group before you can delete the group.