Each character is based om a popular rpg game, and at the start I want the player to have the choice of 6 party members. Two of whcih are compulserary. Now depending on the other characters, certain puzzles may be solved. For instance, very early on, a Beholder ship enters the port of Waterdeep.
The plasyer is then asked if he has the rogue; Sissellnot Smotlebo. If so, then a side adventure begins. Whereupon
Sissellnot may elect to go om a private adventure.

Can you tell me how to go sbout this proess?

Using passages is one way. Like...

@set not rogue
@set not thief

Select party:
[rogue]
[thief]
[[Leave this place]](future code)

[rogue]:
@set rogue
The rogue joins your party.

[thief]:
@set thief
The thief accepts membership to your group.

You can then test later if you have that character, such as ...

{if thief: [[Have the thief steal the key]](stolen key)}