The liability comes from the employer putting them in a certain position and the person using that position to harm someone.
That depends in the US. Vicarious liability means the act of the agent is the same as the act of the principal which means that the agent must be acting as an agent in some capacity for the principal to be legally responsible.For example, a municipality may not be held liable if a police officer dates a female and harms her. Just because he is a cop doesn’t make all his conduct agency conduct. It may be argued that JW elders have express authority from the WT. But it can also be argued that that authority is limited to a spiritual setting. So an entity is not always legally liable for the conduct of someone that works for them or has been given some authority to act.