I just got the book “Bad Blood” about Theranos by John Carreyrou of WSJ. The book might reveal how they were recruited.
In my experience, they have connections with folks who raise investment funds for firms backed with private equity. They also network with each other. And sometimes, they meet someone with an interesting business proposal on their own, and then network back to the rest of the community.
This is a fairly high profile case, but hardly unusual.
The Reason interview with Carreyrou (posted by KC_Lion above in #6 does indeed explain how she landed her first high profile board member and then leveraged that into a powerful board. I doubt that any of them had high-tech board experience and little or no medical device board experience.
She played the femme-fatale / feminine mystic bit very well and Carreyrou says many of these older men were taken in by that.