I got some idea's, but the main question for the easiest way
I got in mind, is:
How manu party members will there be?
because there are some ways you can do it.
if you have 4 members max and 10 outside, than you can make it work
with 2 plugins , and a choice list.
if you have 2 members in party, and want to swap between 4 others, you might
need 1, but if you have a member, it shouldn't be visible in the party.
but the way I go in mind with choices can be one way, if there are better
methodes, probably, but there are always 3 differents ways.
easy way, the average way, and the advance clean way
however you set that up is totally up to you and how conforable you are
but if you are new, I would suggest to learn the system a bit more,
but if you dont know how to event a mechanic, you dont learn much.
there is a command on removing and adding party member, easy way to
swap which is also a methode with choices to swap actors.
swap actor 1 with 2, or actor 2 with new member etc etc.