I'll be mid way through a mission and my group of people will just stop following and also sometimes they stop speaking all together so I have no idea what to do and cant get any further. Any ideas whats going wrong guys?

If you are using any of the cheats, I would quit out of the game and restart the last checkpoint without them. If that's not the case, then you could have a junk disc and should either clean it or return the copy for another one.