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.