When I was still teaching, we had a faculty book club in which the host selected books based partly on whether or not she could recruit the authors to speak. So that was a case of someone reaching out to an author.
I don't know how she developed her pull, but she actually delivered some fairly high-profile authors. (Some lived in the LA area, some had other business in the LA area.) I think perhaps it was that she delivered more than just faculty purchases. She developed a parent book club and also one that didn't have any connection with school. For at least part of the time, she also had an online book club. And when the high school wanted to do a schoolwide reading experience, she was the one who picked the book. In other words, I think maybe publishers liked her because she could deliver a fair number of sales.
Who knows what else might have been at play? She had the self promotion gene that I seem to lack. She only published one article herself, but had she decided to write full-time, I'm pretty sure she would have ended up on the bestseller list, one way or another. She had just that kind of force-of-nature personality.
She also had the community going for her. Lots of entertainment personalities lived in the area. I doubt she pitched this part, but having her feature a book for schoolwide or community reading offered a pretty good chance of getting it into the hands of a producer or director--or at least, one of their kids, who might leave it lying around the house at some point...
But needless to say, she wasn't charging authors for the privilege of being associated with her book club. If anything, I think they might have gotten an appearance fee. But they certainly got red carpet treatment.