without the expanded shadow party hack, there is more difficulty
but the join/departing thing is an event instruction. for example, i made edward join in toria. he just crawls right out of bed and walks into your party lol. a normal game has 5 slots for shadow party. well i guess you know all of that, so there are "tricks" you can do to preserve a certain amount of characters but inevitably some will have to lose all their data.
an example. at mt ordeals you have Cecil Dark Knight, Palom and Porom
Kain, Edward, Rosa, Yang, Rydia are all saved in the shadow party at this time. but I want to save cecil's dark knight stats and load it back when he becomes a paladin. how do i do this when all 5 shadows are full? Well just when you are about to become a paladin I made the game add edward to the party, save cecil in the slot that edward was justin, then load cecil back into his paladin actor (preserving him as a dark knight), then i made the game remove and store edward back in that same slot once again. All the while an event is going on so you have no opportunity to know that any of that ever happened...
thats an example... and it requires a mere sacrifice of a few small and somewhat pointless details of the original event in order to insert the add/remove instructions