BitStream enter;
enter.Write(getSAMPVehicleIDFromGTAVehicle(pVehicle)); // id car
enter.Write(1); //as passenger - http://ru.cleo.wikia.com/wiki/RPC
g_RakClient->RPC(RPC_EnterVehicle, &enter, HIGH_PRIORITY, UNRELIABLE_SEQUENCED, 0, 0);