The QR code is just a machine-readable code that has some information. In this case, your camera reads the code and finds a website for you. You could equally go to that website by entering the long URL address, or by clicking on
this link to the Pilgrim Office website.
Go to the registration site by any of those means, enter your information (for each person, separately), and get your alphanumeric code by email. Then, either star/flag that email, and/or write the code on your credential. When you get to Santiago, walk up to the Pilgrim Office entrance, show them your code, and they will tell you what to do next.
I did this recently. Although one could argue about the traffic flow within the office complex, it was quite straightforward, pleasant, and there were lots of people to help. There was no drama and confusion that some of the threads on this forum would suggest. I don't doubt that there have been occasions, especially in the early stages, when there has been confusion and inefficiency. But it is certainly not as hard as checking into your flight, or buying a Metro ticket for the first time.