Code:
on *:JOIN:#:{
  if ( $nick == $me ) {
    set %ial $+ # 1
    who #
  }
}
raw 352:*:{ 
  if ( $eval(% $+ ial $+ $2,2) == 1 ) {
    haltdef
  }
}
raw 315:*:{ 
  if ( $eval(% $+ ial $+ $2,2) == 1 ) {
    unset %ial $+ $2
    haltdef
  }
}

That will get the addresses of everyone in the channel when you join. Not sure why you have the /who $nick in (the IAL is updated with someone's address when they join anyway).