Having walked and crewed, I can tell you without a doubt that crewing is harder! But it is an incredible experience. All you have to do is go to the3day.org and click on register. There you can register as a crew member or walker. After that, you will have an interview with the crew coordinator and decide where you will be. You may or may not get the area you request (I always tell them that I would be happy to do whatever is needed, and I am)

I know in Dallas, the crew positions fill up quick, so the earlier you register, the better.