All three clubs are secretive about their application process—the special sauce that defines their unique communities.

Magnises doesn't outright reject applicants but will keep some in limbo indefinitely. And it worries about accepting too many applicants from similar backgrounds. "We can't let in 2,000 people from the same company tomorrow, because it would sway the group and the group-think," its CEO McFarland said.

The premium his club places on diversity was evident at its recent event at the Manhattan hotel, where investment bankers mingled with designers and a computer programmer was wrangled into a discussion of Eastern European history.

"I'm ready to pay a premium if the events are more curated and one-of-a-kind."

"There's really no disqualifying factor, necessarily," said Cisco, the Select CEO, of his club. "Employment and income are a factor, obviously, but we feel we're building a community here. We want to make sure anyone who joins would be someone we see connecting with the community at large."

Select's secret sauce has resulted in a membership base with an average income of $280,000 a year—considerably more than the average American earns, and especially impressive for the notoriously debt-ridden millennial demographic.

Someone with that kind of salary could certainly afford to join a country club or university club with high dues. But such a club would offer a different sort of community, and a young member might not be able to take advantage of its perks if they moved, say, from Los Angeles to New York for a job.

The new millennial-centric clubs pride themselves on offering a way for new transplants to meet friends and find hot spots—a de facto best friend for anybody with a new career in a new town.

Neeharika Sinha, a 32-year-old tech entrepreneur, said she joined Magnises after it approached her on Instagram, where she has close to 1,000 followers. (The club said it occasionally reaches out to people it considers community influencers if it thinks they would add to its community, and said such outreach is important to its plans for 1 million members within the next three years.)

"I moved from New York to San Francisco two years back. I kind of felt uprooted, and I felt like I needed to know more events," Sinha explained. She has yet to attend an event, but she added: "I felt this was a good way to connect with the city. I'm ready to pay a premium if the events are more curated and one-of-a-kind."