as far as I know there is no user-group sysop, because there is only one sysop-user per wiki-installation. In my wiki-installation the sysop-user is called WikiSysop (which I think is the default). This user appears in special:userlist, but is not denoted as sysop.
Permissions can't be given directly to users, only to user groups, and users added to groups, so there has to be a sysop group if you are to have any sysops.