AdiIRC

You can try putting this in the Script Editor, then type /savenicks in the channel, the nicks will be saved to nicks.txt.

alias savenicks {
  var %s, %c = 0
  while (%c < $nick(#, 0)) {
    inc %c
    %s = %s $+ $nick(#, %c) $+ $crlf
  }

  write -c nicks.txt %s
}

"/savebuf -l #chan <file>" can be used to accomplish this.