Fourth passenger

Hi,

I am based in Yokohama on a three year researcher visa. My parents and my brother (total three) will be visiting me in September for a month. I would like to take a week long railpass for them to explore japan. I will also have to join them as they are very new to the place. However, I am afride about purchasing tickets to travel in Shinkansen always as I may end up paying almost seven times more than what the seven day pass actually costs, if we travel much. Hence, I would like to inquire is there any way that I could also access the railpass service?