you can also change the

 if (!$2) { msg # 4USAGE: !userlist <channel> }
  elseif ($me !ison $2) { msg # 4ERROR: I can't scan $2 as I'm not in it. }

to

if (!$2 || $me !ison $2) { msg # 4 $iif(!$2,USAGE: !userlist <channel>,ERROR: I can't scan $2 as I'm not in it.) }

but whatever makes no diff....