On a weekday afternoon last fall, roughly two dozen strangers gathered in a Brooklyn loft to talk about a common affliction.

One woman had been struggling with her identity since becoming a mother. Another was feeling out of place at work. Half of the people in attendance admitted they were lonely.

“Our sense of loneliness,” said Radha Agrawal, who was leading the conversation, “is literally making us sick.”

Seated in a circle of beanbags, cushions and armchairs, the group breathed through a guided meditation. They did an active-listening exercise. They stood in a circle and swayed together. At the end, they took a collective inhale and loudly exhaled in unison.

“There’s this chemistry of sadness permeating all of our cities right now,” said Agrawal, wearing a big blue hat and long yellow sundress, and nodding empathetically. “It’s up to us to shift that chemistry.”

Last year, U.S. Surgeon General Vivek Murthy declared loneliness an epidemic , ringing the alarm that social isolation was causing mental and physical harm. Teenagers were reporting record levels of sadness. Suicide rates were at a record high. While Covid kept people from their friends, families and peers, there were other factors at play, which Dr. Murthy described in his May 2023 report—namely the advent of smartphone technology and the rise of social media . Once heralded as innovations, these creations of private industry were suddenly the problem.

Who better to help than investors and entrepreneurs?

Agrawal, best known for co-founding the substance-free, crack-of-dawn rave-throwing company Daybreaker in 2013, stands among the crowd of thought leaders looking to disrupt the social ailment of our era. Belong Center, a new nonprofit she co-launched in December, is one of several ventures across health, entertainment and tech aimed at tackling loneliness. Firms as big as Sequoia Capital are backing some of their efforts.

Like Alcoholics Anonymous for the Burning Man set, Belong Center has free-spirited vibes and aims to heal “mind, body and soul,” says Agrawal. Its meetings attract a Daybreaker-ish crowd—openhearted seekers in cowboy hats and flowy pants—but ask attendees to get a lot more vulnerable.

Its “circles,” as the 60-minute sessions are called, are run by facilitators who follow a curriculum of group meditation, discussion and movement. They have been free to join so far, though the nonprofit plans to start suggesting a donation of around $20. The intention is to help people improve existing relationships and establish new connections.

“It’s a 2.0 community center,” says Agrawal. “Everyone is talking about fitness and nutrition, but people don’t realize meaningful relationships should also be seen as mandatory personal care.”

Agrawal has an expansive outlook and a true conviction to fix the world. She says she felt a “dharmic calling” to take on loneliness. She speaks about the nonprofit she chairs with confident and earnest energy.

She says she has lofty goals for Belong Center—not to be confused with Belong Institute, a $2,000 online course she offers to entrepreneurs, or Belong , her 2018 book on belonging.

Belong Center will pop up in a dozen cities around the U.S. this year, she says. There are also plans for an apparel line, a Sunday school for kids and a TV show that teaches America how to make friends.

“We are going to build an entire network across the industry and globe,” she says. “I just visited a refugee camp in Kenya, and we just confirmed opening up a Belong Center at the refugee camp.”

Elie Reiss, a 32-year-old product manager living in Brooklyn, says he attended a Belong Center circle in December because he had been searching for a nonalcoholic social space. “It was nice to have a place that isn’t for drinking or networking to find a business partner, where you can pause to take a breath,” he says.

Reiss says the experience didn’t leave him with a stronger sense of belonging, but he says he might feel different if he went a few times. The circle he attended didn’t feel awkward, he says, as the participants were so dialed in.

Investor money is fueling other startups that aim to strengthen friendships and provide community. The founders of SoulCycle received $7.2 million in funding from Maveron, the venture-capital firm co-founded by Howard Schultz , for its new wellness center Peoplehood, which runs guided group conversations aimed at improving relationships. Former Tinder CEO Renate Nyborg has raised $4.9 million from investors including Sequoia to make Meeno, an AI chatbot that helps people practice difficult conversations.

Agrawal says Belong Center has secured about $750,000 of committed funding from donors including Sam Ben-Avraham, co-founder of sneaker store Kith, celebrity mycologist Paul Stamets and Monty Moran, a former Chipotle executive. Its board members include the physician and social-media personality Mark Hyman; former CEO of period-underwear company Thinx (and Radha’s twin) Miki Agrawal; software entrepreneur Ping Fu, who is also on the board of Burning Man; and the chef and philanthropist (and Elon’s brother) Kimbal Musk .

In an email, Musk says he believes in the Belong Center mission and in Agrawal.

“I have experienced loneliness in my life,” Musk says. “Those are some of the most difficult periods I remember.” He says he joined the board to combat a problem that has never been more pressing.

In a 2023 Meta-Gallup survey of people from 142 countries, more than half of respondents described feeling at least a little lonely. Nearly a quarter of respondents said they felt very or fairly lonely. A 2021 report from the Survey Center on American Life found that 15 percent of men today don’t have close friendships , a fivefold increase since 1990. One in five men in the U.S. who aren’t married or in a relationship said they didn’t have any close friends at all.

Lucas Krump, the CEO and co-founder of Evryman, a men’s retreat and online community, says society discourages men from expressing vulnerability, which can limit their ability to form deep bonds as they age. “Men tend to create friendships early in life through shared experiences, school, sports, fraternities, first job,” he says. “As life progresses into our late 20s and early 30s, we tend to lose touch.”

Natalie Dillon, a partner at Maveron, believes there is a willingness to spend if it means feeling less alone. “You already see that with the amount of money people are paying for Tinder premium or Hinge premium ,” she says. “People are desperate to find connection.”

Dillon believes it’s too early to determine the potential market size, but says loneliness will “1,000 percent be a category investors put money into and that companies build,” she says. “It could be just as big as mental health, if not bigger.”

In a phone interview, Dr. Murthy, the surgeon general, said he believed loneliness had major public-health implications. He says people struggling with social disconnection could be at risk for depression and anxiety as well as heart disease, dementia and premature death. Loneliness costs employers about $154 billion a year because of stress-induced work absences, according to Dr. Murthy.

“ Teams no longer know each other ,” says Julie Rice, the co-founder of Peoplehood. “People are trying to collaborate without having any understanding of who is on their team.” Peoplehood offers 60-minute sessions at its New York City location ($35) or virtually ($25), where participants can learn how to build high-quality human connections. Rice believes that many of us are seeking ways “to be better at people-ing.”

Organized religion is also in steep decline in the U.S. Belong Center’s vibes are spiritual but irreligious and a bit woo-woo—a salable proposition for a secular, wellness-minded era.

“The role of the church, or the community center, has not kept up with the times, and there’s a void,” says Michael Clark, a real-estate developer and philanthropist who donated to Belong Center. “Having a place that feels like a small village feels like an obvious addition to the city.”

Brian Hawkins, a 54-year-old stylist from Norwalk, Conn., wasn’t experiencing loneliness when he showed up to a Belong Center circle. He’s a Daybreaker fan and wanted to support the cause. But the experience ended up being helpful, he says. The group did a conflict-resolution exercise, and he practiced a mock conversation with a friend he’d had a falling out with. “It was a good way of releasing and finding other people to release to,” he says.

Agrawal hopes to build a network of businesses that will host these circles regularly in their offices. She says Wonder Sciences, a psychedelic mental-health-care startup, has committed to donating its corporate social responsibility budget to Belong Center, and she intends to ask other companies to do the same.

The company plans to launch a marketing campaign featuring bright yellow benches in public parks, Agrawal says, which invite people to sit down and connect.

Agrawal has a track record of getting people in Miami, San Francisco, Austin and New York to show up for the inconceivable—like a 5 a.m. sober dance party—and actually enjoy it. Can she get the masses to come to what is essentially group therapy?

“I’m really not worried,” she says. “People are so lonely, they are going to find us.”

Write to Chavie Lieber at Chavie.Lieber@WSJ.com