well if you know which party members the player would have then simply using an event for adding and removing party members works well enough. but if this is for a part of the game where the player can return later with more party member than when they first arrived then here's what i did.
i made a common event called Anna Alone which does the following.
conditional branch.
if party member Astrid is in the party.
remove Astrid
add variable, party member Astrid +1
i then follow up this event with more conditional branches for every party member.
with this Astrid gets removed from the party and a variable is created to keep track of the fact that she's only supposed to be temporarily removed.
then i created another common event called Anna Not Alone. here's what this one does.
conditional branch
if variable, party member Astrid = 1
add Astrid
add variable, party member Astrid -1
with this Astrid gets added back into the party if her variable says that she's been temporarily removed and her variable gets reset to 0. i use this to create areas where Anna goes in alone yet the player might return later in the game with more party member than they first had when reaching the area.
after doing this you simply have to expand these common events to account for every party members. so you don't need a plugin to do this, you just need to get creative with the event system.