Hi, Maria Jo...
When looking at transport options from Pamplona to SJPdP, I found
this taxi company in Pamplona. They specialize in transporting pilgrims from Pamplona to Saint Jean Pied de Port.
You make your reservation online, by email, or by phone. You tell them your arrival location for pick up. During the reservation process,
you can choose an option stating that you want to share a taxi with others going to SJPdP. The fare is shared by all the passengers. The company will then work to schedule a full taxi for you.
If they already have a taxi scheduled to travel to SJPdP near your time of arrival in Pamplona, and that taxi still has a seat available, they can reserve a seat on that taxi for you.