If you are the primary policy holder and someone else drives the car and gets into an accident is it your fault or theirs?

Fault does not really matter, the claim will go under the policy that the vehicle is insured under as long as the driver of the car had the owners permission to drive the vehicle. Be careful though, some policies are "named driver" policies and if the driver of the car was not listed in the policy, there may not be any coverage. If that is the case, and the driver has their own policy, you can try to attach to that policy.