Conan and the others guests bear no more responsibility for what happened to the Reiners than a bystander who refused to intervene while witnessing a child’s kidnapping.
Rob and Michelle were adults. They had more experience dealing with Nick than anyone else at the party. They could have gone to a hotel, phoned the police, then gone home with the police to arrest Nick.
Rob and Michelle were always in control of their actions. They chose to return home and go to bed.