The thing is though, they want to select the nicknames upon joining and with your code they'd need to manually select the nicknames then proceed with the command.

How about something like,

Code:
on me:*:join:[color:red]#channel[/color]: {
  getnicks
}

alias getnicks {
  if ($active ischan) {
    var %x = $nick($active,0)
    while (%x) {
      sline -a $active %x
      msg $nick($active,%x) Hello $nick($active,%x)
      dec %x
    }
  }
}