If you don't find someone to share with online, when you arrive at the Pamplona airport, you will see other pilgrims (or perhaps if you are flying in, you will spot them in the departure lounge at your departing point - very easy to spot!) and you can ask them to share. Taxis are always waiting at Pamplona Airport. In the Barcelona airport, my friend and I found 2 other pilgrims to share the taxi once we would arrive in Pamplona, $110 Euros total delivered each one of us to our respective hostal. (We paid him 120 Euros because he was such a nice driver)