I try make it short,
when you reach start of chapter 2 don't do anything before speaking with John,Abigail,Jack and Sadie.
and do not do uncle mission first(you can do it as second mission).
Otherwise you might lose those 4 characters for the entire of the game( beside scripted missions).
EDIT 1: I see many people asking how to fix this. So here are the steps:
1.Save at start of chapter 2.
2.Talk with John,Abi,Jack and sadie.
3.Sleep until night to make sure they are still there
4.Do any mission EXCEPT UNCLE mission.
5.go back and see if they are still there.
6.Enjoy the game.
Now I'm not sure if that's the fix but it worked for me the 2 times I test it out.
Edit 2:For people saying they are not supposed to be in camp I have played chapter 2 with and without them and there is huge difference.
Edit 3: People are saying it get fixed at chapter 4, since I played chapter 2 twice and did testing for bug I haven't reached there yet but I believe them.
Edit 4: Apparently they are writing articles about this issue now so it won't be long before rockstar will address this issue.