Well, he's using a party switcher script. Anyway, do this:
Use four in-game variables, whatever IDs are free. For the purposes of this, I will let A, B, C, and D represent the four variables IDs you chose.
Name them Party Member 1, Party Member 2, Party Member 3, and Party Member 4
Then, use the Script event command and put in it:
actors = $game_party.actors
$game_variables[<A>] = actors[0].id if actors.size > 0
$game_variables[<B>] = actors[1].id if actors.size > 1
$game_variables[<C>] = actors[2].id if actors.size > 2
$game_variables[<D>] = actors[3].id if actors.size > 3
Then you can use conditional branches on Variables A, B, C, and D to discover which member is currently in the party.