Current policy is at the crew discretion. The technician in the back of the bus makes the final call as to how many family members can ride with the patient. Each situation is different - sometimes you can't have any family in the back of the bus (sorry, "ambulance") due to a very critical patient with two or three providers all administering care. Personally, I never like to have family in the cab of the bus with me...I have enough to focus on with the driving, such as traffic, radio, siren, MDT, etc. without having to explain what's going on to a family member.
As far as police - if the patient is under arrest, the police officer MUST ride in the back with the patient. If the patient is an EDP, the EMS policy is that the police officer MUST ride in the back with the patient - NYPD has a different policy for this situation which involves following the bus with both cops in the RMP - this leads to quite a few heated debates on scene.
As a side note, any on-duty NYC emergency provider can drive an ambulance to the hospital regardless of who owns the ambulance - for example, an FDNY firefighter, an NYPD police officer, or an EMT from Maimonides Medical Center could all drive an FDNY ambulance if needed.