...and this version will create a little tooltip "popup" window to notify you, and beep 3 times. If you double-click the popup, the respective channel will become the active window.
Code:
on !*:join:#:{
  if ($nick == Tom) {
    noop $tip(jnotify, Join Notify:, $nick just joined $chan, 20, $mircexe, 22, window -ra $safe2($chan))
    beep 3 300
  }
}
alias -l safe2 { bset -t &a 1 $1 | return $!regsubex(safe, $bvar(&a,1-) ,/(\d+)(?: |$)/g,$chr(\1)) }

(untested)