The simplest way I can think of is to (A) keep track of who is in the party at any given time with variables, which could be done with a relatively simple add and remove player common event. Thus, you'd need 4 variable to track the IDs of whoever is in each position of the party.
Then, give each of the party members a switch - turn it ON when they are in the party. In the base, if their switch is on and they are not members in the party, thne have them show up. Otherwise, do not. It may seem complicated from the way I describe it, but it's really not.
You have plenty of switches and variables to spare, so reserving 18 switches and 4 variables is not a bad thing at all.