Ideas like this have been bandied about for quite some time, but I didn't see
any bugs open for it, so:
Often wikis will want custom groups. Unfortunately, these groups cannot
currently be assigned easily. Extensions have been created to allow bureaucrats
to assign and remove certain privileges, but there's no generalized way to allow
them to give and remove specific groups.
The ideal would be that in $wgGroupPermissions, the 'userrights' permission
would be changed to an array, so you could set something like
$wgGroupPermissions['bureaucrat']['userrights']['sysop']['add'] = true;.