the reserve is simply any party member above number 4, and you can use the regular change membership commands for those actors.
however I wouldn't bother with that and instead go to check the process before adding members.
on the HQ map, you'll make one event placeholder for every party member. you'll need three or four event pages there depending on what you want to do.
first page is when the player doesn't know of the actor - empty page, empty chair/bed or whatever.
second page would be conditioned to a switch "actor available", having the actor picture for the actor staying at HQ and asking if he should join the party. And have that joining conditioned on space in the party.
third page is conditioned on actor in party and basically an empty chair again, possibly asking if the actor should leave here or giving options on what the actor can do in the HQ or whatever.
then on every event in the world where the player can find actors, he will activate the switch for "actor available" and check for party size before adding the actor. If the party is already too large the actor will automatically be available on HQ after the switch.