The easiest way to go about this is using mirc's user list:
Code:
menu nicklist {
  Add $$1 to Voice:.auser -a voice $$1 | echo 2 # * Added $$1 to voice list.
  Remove $$1 from Voice:ruser voice $$1
}
on voice:TEXT:*:?: {
  if ($strip($1) == !working) && ($nick isin $ulist(*)) {
    pvoice 1 #YourChannel $nick
  }
}
Right click on your pop ups nick list to select people you want to add to your voice list, as well as the one you want to remove. Those added will be able to trigger the script upon !working. Those removed won't be able to anymore. Change #YourChannel to your actual channel name.