At the black-tie opening of Casa Cruz on Manhattan’s Upper East Side, Daphne Guinness, the English heiress and fashion muse, paused like a monarch butterfly on the third-floor landing in a cascading crystal robe of her own design.

“Shall we go up?” she asked her son, Nicolas Niarchos. “Or shall we go down?”

That was the reigning question of the night as well-heeled guests explored the six-story Beaux-Arts mansion at 36 East 61st Street. The hype and anticipation had been mounting: The Wall Street Journal referred to it as “the buzziest slice of London’s nightlife” and Vogue called it “New York’s most glamorous new restaurant.”

After years of Covid delays, Casa Cruz New York — the sister of the London hot spot that has drawn Sir Elton John, Prince Harry and Mick Jagger — held a V.I.P. preview on Sept. 9, timed to New York Fashion Week and the Armory Show. Never mind that the Queen had died the day before. The chance to run around the city’s latest exclusive playground titillated the 300 invited guests.

“The place is absolutely brilliant,” one said aloud on the mirrored staircase inspired by Coco Chanel’s Paris shop, creating an elegant opportunity for infinite self-regard.

To admire: the copper detailing, Brazilian cherry paneling, turntables with vintage LPs, Boteros, Warhols, fireplaces and walls upholstered in Casa Cruz green corduroy — the same propriety shade as the double crepe staff dresses by Emilia Wickstead.

“She designs for the Princess of Wales,” one knowing guest said.

Then there were the people — a fizzy cocktail of the international elite. They admired each other and snatched foie gras canapés from silver trays. Dasha Zhukova and Lauren Santo Domingo huddled at a table under the restaurant’s pink awning. Victoria von Faber-Castell, a young pencil heiress, lounged with Isabella Massenet, the Net-a-Porter scion, and Flynn Busson, the son of Elle Macpherson.

Elizabeth Saltzman, a celebrity stylist who lives in London, eyed the Marlboro Lights and branded lighters left out for guests. “I’m dying for a cigarette but it feels rude to smoke indoors,” she told Juan Santa Cruz, 51, the host, ringleader and enabler behind the scene.

“Go ahead and smoke wherever you like,” Mr. Santa Cruz said. “Everyone can misbehave.”

Does New York need more private clubs that cater to the rich? It started in the pre-Instagram dark ages with Soho House, which opened in the meatpacking district in 2003 and has since grown tentacles into the Lower East Side and Brooklyn.

The Core Club in Midtown, which courts a wealthy careerist crowd, followed, then NeueHouse and the now-defunct Norwood. Like gossip and wildfire, the trend has accelerated since the pandemic, with a half-dozen new private clubs and restaurants.

Zero Bond has been drawing the young and casual, along with Mayor Eric Adams, to its art strewn NoHo loft. Casa Cipriani hugs the Battery along Manhattan’s southern tip like an old-world ocean liner, with members clutching status handbags and phones as if they were life preservers.

The Aman New York in Midtown has a private jazz club, and the Fasano Fifth Avenue has duplex apartments overlooking Central Park. An offshoot of 5 Hertford Street, another London import, is slated to open on the Upper East Side. And San Vicente Bungalows, a private Hollywood clubhouse, plans to open at the former Jane Hotel in the West Village.

How much more exclusivity can one city take?

Most of these clubs charge between $4,000 or $5,000 in annual dues, but Casa Cruz is different. It is technically not a club, but a restaurant with an investor group of partners that pay between $250,000 to $500,000 to join, according to Kate Bartle, a spokeswoman.

The 99 current investors get exclusive access to the fourth floor and rooftop terrace, where there are lounges and dining rooms, accessible by a private elevator. The main restaurant and lounges on second and third floors are open to the public, assuming one can get a reservation.

“And when partners invest in something, they end up promoting it to their friends, so you don’t have to do any marketing” said Mr. Santa Cruz, who left a career in investment banking a dozen years ago, and has three restaurants in London and one in Buenos Aires. “To be successful, I don’t have to attract that many people.”

Every potential partner is vetted by him and an informal tribunal of friends. He said he wants the kind of investors around who stay off their phones, and should know how to rub elbows without rubbing others the wrong way. And of course, they should know how to be nice to the staff.

“But I see myself less as a dictator than a director,” Mr. Santa Cruz said over dinner at Harry Cipriani a few days before his opening. “That means I oversee the design, the lighting, the music, food and find the kind of people who know how to act.”

An impeccably mannered man with a resemblance to a blue-eyed George Clooney, Mr. Cruz was born in Chile and grew up in Uruguay and Switzerland. His mother, Maria Ducci, was chief of staff of an international labor organization and his father, Juan Santa Cruz, oversaw the family’s agricultural properties. His great-grandfather and great-uncle were both Chilean ambassadors to England and his aunt, Lucia Santa Cruz, was a friend of King Charles.

“He’s got a va-va-voom vibe when he walks in a room that makes it electric,” said Peter Hawkings, 48, the senior vice president of men’s wear at Tom Ford. “He’s just a lot of fun.”

“He’s the mayor of Punta Del Este and everyone loves him wherever he goes,” said Jamie Tisch, 54, the sociable philanthropist. “And that makes his places a success.”

In 2016, he briefly opened a pop-up restaurant called Casa Cruz in TriBeCa, and the enthusiastic response encouraged him to find a permanent place in the city. “New York is so open and positive,” he said. “People here don’t want you to fail.”

Some social observers are not pleased about the elitist model of socializing that Casa Cruz and members-only clubs represent.

Steve Cuozzo, a columnist for The New York Post, recently called private clubs a “cancer on the city” where “dining in private places is reserved for the privileged few.”

Euan Rellie, 54, a sociable investment banker and London transplant, thinks that they run the risk of being stuffy and boring. “Who wants to go to a place that only has rich people?” he said. “I want a few billionaires, then some artists, fashion people, writers and athletes too.”

Ann Dexter-Jones, another London transplant and jewelry designer, questions if having cash is the same as having caché. “It used to be how interesting you were, now it’s how much you can pay,” said Ms. Dexter-Jones, who declined to give her age. She frequents private clubs in both cities but belongs to none herself. “If you’re cool you don’t have to join,” she added, “you just get invited.”

That was the case at the black-tie opening, a party painstakingly curated by Mr. Santa Cruz to include a mix of young, old and in-between. They included investors and other friends of Mr. Santa Cruz. As the night wore on, they made themselves at home, drinking, carousing and smoking. Prince Achileas-Andreas of Greece and Denmark, 22, was bemused when an over-served guest filched four Marlboro Lights from his personal pack on a table.

“Would you like some help?” he asked politely.

Many guests had flown in expressly for the party as a show of allegiance. French, Spanish, Greek, Italian and other languages peppered the intimate lounges.

“There are some very good South Americans here,” said Anastasia Saravia, 26, who works in client relations for H Huntsman and Sons, the British men’s wear company.

Hannah Bronfman, 34, a wellness entrepreneur, influencer and heir to the Seagram fortune, admired the multigenerational scene. She thought the neighborhood needed a cool place to go. “It’s all very London,” she said.