I've done two local ragnars, in 2012 and 2013. My team found me - someone in the group knew I was a longtime distance runner and approached me to join. It was a pretty neat experience. I never would have considered running at night by myself, but in that environment, I was able to pull it off.
You can go on the race website and look for teams that are looking for runners - often teams will have someone drop out before race day due to injury or a work conflict.
You need to be able to do 14-22 miles between 3 runs over about a day and a half. it's also helpful to be able to function on little sleep for short periods of time, and to be ok not showering right away after a run.

One good way to get a taste for the running involved, which I did before each one: run 5 miles tonight just before it gets dark. Get up early tomorrow and run another 5 miles. Run another 5 miles as soon as you get out of work tomorrow. See how you feel.
It's also helpful to be flexible and adaptive because your assigned legs of the race might change last minute and you'll go from doing 3 5-mile stretches at normal elevation to doing one 8-miler up a mountain, a 4-miler in the middle of the night and 5 miles downhill. that kind of freaked me out the first time.
It was overall a great experience for me, and cemented one previous friendship while making a whole set of new friends with whom I'm still in touch.