If someone else drives my car and gets hit from behind what will happen?

Well generally, any time a car hits another cars tail end, it is that persons fault. (Usually because they say that the person was not paying attention and did not stop in time to avoid hitting the other car.) However, if the insurance is not also in the name of the person you are letting drive you car, and the person that hit them claims that they(person who got hit) caused it, you car could get towed (about 200 dollars for that) and their could be a trial. But you would not get in any trouble, unless that $200 was for an anniversary present.