well, that would kind of be awkward anyway, so I'll tell you. To add an actor, just do this:
$game_party.add_actor (actor id)
and to remove:
$game_party.remove_actor (actor id)
So now, all you have to do is preserve the original $game_party.actors in an array (this is probably the most difficult part of the script, surprisingly), and then provide a selection based on this array (so the names of all in the party), and what occurs when you select each name can essentially be reduced to a comparison:
if actor in $game_party.actors, remove, if actor not in $game_party.actors, add.
That'd be my idea at least. You might need to adjust his menu script a little to switch dependence from $game_party.actors to your new array. If you decide to do it and run into any problems, don't hesitate to ask for advice.