75% are active-duty or former military (all O-4 or higher), and most are graduates of one of the service academies. Without taking anything away from their accomplishment, it's safe to say these individuals were already high-achievers to begin with.
Ps. I love the verb grok. Such a great book.
A civilian would have to complete training equivalent to that of a military pilot before actually being cleared for a mission. One-third of military pilots don't, though it's entirely possible the civilian could. However, it probably makes more sense to train those individuals who have already shown they can perform under similar conditions.