It is not necessary, per California smog inspection procedures, for a vehicle's owner to be present during the smog inspection; however smog stations' are independent operators, each might have it's own requirements due to insurance, standard operating method, available parking, so on.
Generally speaking, you will not have a problem dropping off your vehicle and returning to pick it up after the smog check is complete. Check with the shop manager or owner. Ensure they will allow you to leave your car there until the smog is complete. Ensure you agree on a time you will come back to pick up your vehicle, as probably one of the more important factors will be parking space availability.
