Prosecutors must prove that the person they charge with Driving Under the Influence is the one who is in operational control of the vehicle. If they cannot provide that proof, the defendant’s lawyer can make arguments for an acquittal at trial or for the complete dismissal of the charges.
If there are multiple people in the car, but at the time the police officer approached the car everyone is outside of the vehicle and the police officers cannot establish through evidence which person was the one driving then the police officer will not make an arrest. This is why it is always important for people to remember they have the constitutional right to not answer questions when asked by police officers.