The
wiki tutorial is a good starting point.
As it stands, the outcome of the tutorial is under player control, so they can use the Remove All button to clear the party picker, then pick the new party member (or not). That's a tried-and-tested method.
If you'd like to experiment, search the official campaign scripts (using a utility such as
DAToolChest)) for other examples.
I imagine that removing all party members by script involves changing the follower status to FOLLOWER_STATE_AVAILABLE and tidying up the party-related plot flags.
You'll notice that when a party member is hired in the tutorial, their status is set to FOLLOWER_STATE_ACTIVE. Another option is FOLLOWER_STATE_LOCKEDACTIVE, which prevents the player from deselecting the new member.
Hope that helps - I haven't played around with the camp, so maybe someone else can help with that.