(Ari Perilstein/Getty Images for American Express)

When my friends Katy and Nathan moved to Boston in 2014, they joined November Project, whose members gather for free early-morning workouts in cities around the country. It didn’t take long before they were hooked — on the workouts and the community.

Like an organized religion without the religion, Nathan says, November Project offered a network of people with shared values, including interests in good health and friendships as well as a willingness to embrace such rituals as hugs and chants. “For me,” he says, “it’s essentially the Church of Playing Outside and Working Out.”

It may sound almost cultlike — a description that some November Project devotees embrace. And they’re not alone. Options for group exercise with ritualistic twists and devoted followers include CrossFit, SoulCycle, Bikram Yoga and Fit4Mom (along with plenty of gyms and studios that sell fitness alongside promises of fun and friends).

For some, a tribe-like atmosphere keeps them coming back for more. But others fail to get hooked, and then drift away.

It’s not clear what separates these joiners from those who’d rather not, and that mystery echoes a bigger question facing both fitness studios and public health experts: Why do so few people stick with exercise?

Despite national guidelines that recommend 150 minutes of moderate activity each week for major health benefits plus strength work such as weights or push-ups, only about half of American adults get enough aerobic exercise, according to the Centers for Disease Control and Prevention.

Nearly 30 percent get no physical activity in their spare time.

Even when intentions are good, about half of people who start exercise programs drop out within the first six months, says Rodney Dishman, an exercise scientist at the University of Georgia in Athens. After two years, Dishman says, 80 percent have given up. Those numbers haven’t budged over three decades of research, he adds.

Researchers have found plenty of reasons for quitting, including waning motivation, lack of easy access to exercise facilities or walkable neighborhoods, and false expectations about how quickly results will appear. Injury and discomfort are other common excuses, says Jack Raglin, an exercise psychologist at Indiana University in Bloomington.

To get folks to keep coming back, many fitness studios work to create an enticing environment, often with an emphasis on community, even peer pressure, and competition.

SoulCycle’s website promises empowerment to clients, claiming it doesn’t just change bodies, “it changes lives.” November Project has a page on its website for teasing people who said they’d come but then skipped a morning workout. And in Minneapolis, a studio called The Firm boasts: “We make working out an event, driven by pride, passion and love, building community one name at a time.”

There’s no publicly available data to show whether the community-building — or the guilt — works. But social support can be a powerful motivational tool, some research suggests.

Raglin’s research, for example, has found that, when people enrolled in an exercise program with a spouse, more than 90 percent stuck with it for a year — compared with slightly more than half of those who enrolled alone. When one member of a couple drops out, though, the other usually does, too, Raglin says. That echoes other research showing that friends and family can enhance or sabotage exercise rates.

And not everyone responds the same way to social pressure. In a 2016 study, British researchers found that CrossFit members reported a greater sense of “community belongingness” than did people who went to traditional gyms. But overall, the two groups exercised the same amount, suggesting that people who like to exercise with others may simply seek out more-social gyms.

“It either works,” Raglin says, “or it backfires.”

I have heard both positive and negative stories about group exercise experiences from friends, including plenty of CrossFit fans and some who tried it but eventually moved on. One thought she’d love the workouts but found that she was comparing herself to a mostly younger clientele who spent lots of time together outside the gym — without her.

“It felt,” she says, “like grade-school gym again, frankly.”

And while injury rates are not tracked, risks can rise when people jump into new exercise routines, Raglin says, especially for high-intensity workouts. Even elite athletes are careful to plan recovery and rest days.

“If your goal is partly recreational and partly stress relief and partly enjoying yourself,” he says, “then focusing exclusively on high-intensity stuff may not be the right recipe.”

Relying too much on a group for motivation can also set people up for failure if an injury, busy schedule or social situation distances them from the group, Dishman says.

The same goes for other motivators, he adds, such as the desire to lose weight, live longer or improve heart health. Exercise for far-off goals takes too much time to keep many people going.

Instead, Dishman argues, physical activity is most likely to become a lifelong habit when the desire to do it becomes part of a person’s identity — when motivation comes not from guilt but from a feeling that “I’m exercising because I see myself as an active person.”

A social group or studio membership may provide a pathway to that identity, Dishman says. Eventually, though, satisfaction should come just from moving your body.