The invitation sent by a vivacious actress well-known to TV audiences must have sounded tempting.

In it, pretty, blonde Allison Mack — star of the Superman TV series Smallville — encouraged other celebrities to join an all-women self-help and mentoring group.

Playing on the feminist values of stars including Emma Watson and singer Kelly Clarkson, 35-year-old Mack was wildly enthusiastic about a 'human development and women's movement' that had done so much for her own personal growth, the Daily Mail reported.

Smallville actress Allison Mack leaves Federal court in the Brooklyn borough of New York. Mack is under indictment on sex trafficking charges. Photo / AP
Smallville actress Allison Mack leaves Federal court in the Brooklyn borough of New York. Mack is under indictment on sex trafficking charges. Photo / AP

"As a fellow actress, I can relate so well to your vision and what you want to see in the world," Mack gushed in a tweet to Emma Watson.

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Neither the Harry Potter star nor Clarkson took up the offer — so were spared what could have been a deeply unpleasant experience.

As is now being laid bare in a New York court case that is transfixing America, Allison Mack was allegedly the chief recruiter for a sex slave cult.

Far from gathering for earnest debates on female empowerment, members were allegedly expected not only to have sex with their female 'masters' and a male 'grandmaster', but even to help brand each other with their leader's initials in a horrific act of mutilation.

In an era of the #MeToo movement and a fierce backlash against predatory men, the idea that attractive young women — including well-known actresses, models and billionaire heiresses — could willingly become sexual chattels has stunned America.

Allison Mack has been charged with sex trafficking. She is on $5 million (£3.7 m) bail and under house arrest.

Prosecutors say Mack was the second-in-command of a sorority dubbed DOS (derived from a Latin phrase and loosely translated to 'Master of the Obedient Female Companions'), which existed within a self-help group called Nxivm.

The latter — pronounced Nexium — was set up 15 years ago by a charismatic self-improvement guru named Keith Raniere and was operated, say prosecutors, as a means to subjugate women.

Raniere, 57, has also been charged with sex trafficking after he was arrested in Mexico. He is currently behind bars, awaiting trial.

He and Mack — who played Superman Clark Kent's sidekick Chloe Sullivan on the long-running television series Smallville — deny the charges.

According to the FBI, Raniere's followers call him The Vanguard. He not only rakes in their money but also commands their sexual devotion.

Court papers say he 'maintained a rotating group of 15 to 20 women' who 'are not permitted to have sexual relationships with anyone but Raniere'.

They say Nxivm teaches "the need for men to have multiple sexual partners and the need for women to be monogamous".

His sinister group includes the daughter of Dynasty star Catherine Oxenberg, and the ultra-wealthy stepdaughters of British actor Nigel Havers.

The latter, Clare and Sara Bronfman (whose mother, Georgiana, is the daughter of an Essex pub owner and former wife of one of America's richest men) are the heirs of the Seagrams drinks fortune.

They have reportedly given more than US$150 million ($214 million) to Raniere, allowing Nxivm on one occasion to hire Sir Richard Branson's Caribbean island for a ten-day retreat, buy a private jet and even its own island in Fiji.

The sisters' late father, Edgar Bronfman Snr, who died in 2013, once took Nxivm self-improvement classes but later condemned it as "a cult".

Clare Bronfman, 39, a former showjumper who is now reportedly running Nxivm, insists neither she nor Raniere have done anything wrong.

She says he is "dedicated to the betterment of the lives of others". Although she says she is not a member of the sorority, she claims it has "truly benefitted the lives of its members".

Cult members were branded in a painful ceremony. Photo / Supplied
Cult members were branded in a painful ceremony. Photo / Supplied

Many of the women who joined it, however, vehemently beg to differ.

In court testimony and in interviews, they have described being sucked into a brutal and oppressive regime run by the pudgy, 1.65m Raniere, who used to sport a Jesus beard and John Lennon spectacles, until he cleaned up his image.

Raniere is few people's idea of a sex god. He is, however, fiercely intelligent, and has even been able to convince some of his fawning devotees that he doesn't drive because his intellectual energy sets off radar detectors.

He is severely cross-eyed, too, which former Nxivm publicist Frank Parlato tells me, gives him a "hypnotic stare".

A former girlfriend, Toni Natalie, once described Raniere in federal court as "a compulsive gambler, a sex addict with bizarre desires and needs, and a con man who specialises in Ponzi schemes".

Certainly, his chequered history goes back decades.

The only child of a ballroom dance teacher, prior to setting up Nxivm — which has claimed a total of 16,000 members — Raniere founded a discount group buying club that he shut down after 23 states and two federal agencies launched investigations into allegations that it was a pyramid scheme.

In the late 1990s, he set himself up as a professional and personal development guru, providing New Age self-improvement coaching costing up to US$1,000 ($1425) a day.

The DOS sisterhood said to have been under his control — which he started early last year and which New York prosecutors now describe as an "organised criminal group" — had a structure that let "slaves" become "masters" if they recruited six new slaves.

Prosecutors say recruiters often targeted women facing personal difficulties.

Initially, there would be no mention of "slaves" or "masters", but new members would be warned that the pain and hardship they would face was essential in toughening them up.

They were told that together they would constitute a sort of female version of the Freemasons, dedicated to improving the world.

When they joined, slaves were told to hand over compromising material such as sexually explicit photos or reveal damning stories (true or untrue) that would be made public if ever they were disloyal to the sisterhood. Some signed over all their assets.

Slaves would only learn there was a man in charge of the organisation when they were deeply involved in the group.

By then, they would have been subjected to a grim health regime designed to give them the rake-thin physique that Raniere found attractive.

Many would be forced to run up to 64km a week, take ice-cold showers and go on virtual starvation diets, in some cases consuming just 500 calories a day.

Each woman had to carry a special "slave phone" on which they had to reply within 60 seconds to a message from their female "master".

Former members say that sleep deprivation — brought on by being called by phone at all hours with their "master's" demands — further exhausted them and served to make them more malleable.

As well as performing chores, they would be required to have sex with Raniere. In one vile email revealed in court papers, he is said to have messaged one of his slaves urging her to recruit another woman.

"I think it would be good for you to own a f*** toy slave for me, that you could groom, and use as a tool, to pleasure me," he told her.

One ex-slave told prosecutors she repeatedly met Raniere for sex in the "library" of his mansion near Albany in New York state, where there was a whirlpool bath and a bed.

Some female members moved into the mansion in order to be closer to Raniere. Others lived in the area and used a former restaurant, Apropos, as a meeting centre.

The alleged cult's rituals, such as the brandings, would take place in the homes of various "masters".

Actress Catherine Oxenberg, left, with Stanley Zareff and Toni Natalie, talks to the press following the arraignment of NXIVM leader Keith Raniere in federal court last month. Photo / AP
Actress Catherine Oxenberg, left, with Stanley Zareff and Toni Natalie, talks to the press following the arraignment of NXIVM leader Keith Raniere in federal court last month. Photo / AP

According to court papers, another former "slave" alleged she had once seen a message exchange on a phone owned by her "master", in which the latter had sent Rainiere nude photos of various new slaves.

He wrote back: "All mine?" with a smiling devil emoji. Under-performing slaves were threatened with being smacked with a paddle or put in a cage, say court papers.

By the time they reached the stage when they were branded, insiders recall, they were mental wrecks far too mentally crushed to resist or even complain.

Positively medieval in its brutality, the ordeal seems hardly credible. Yet there are pictures to prove it.The "slave" would be summoned to her "master's" home, blindfolded and led to a candle-lit ceremony on a massage table.

Ordered to strip, she and four other fully naked initiates would take it in turns to hold each other down on the table to receive what they had been reassured would merely be a "small tattoo".

Another slave would film it to provide further blackmail material. When it was their turn, each woman would have to say: "Master, please brand me, it would be an honour."

A family doctor who was allegedly in the sisterhood would use a red-hot cauterising tool to brand a two-inch wide symbol composed of Raniere's initials into the skin near their pubic region.

The stench of burning flesh was so intense they had to wear surgical masks for a procedure that would involve no anaesthetic and take up to 30 minutes.

"It was like a bad horror movie," an actress named Sarah Edmondson said of the perverse ritual she underwent last year.

"We were crying, we were shaking, we were holding each other," she recalled. "It was horrific. I felt petrified. I felt, every part of my body was like: get out of here. Run."

Former Nxivm publicist Frank Parlato, who first exposed the branding, believes he knows of 54 women scarred in this way.

Despite such shocking abuse, some cult members remain loyal to Raniere. Catherine Oxenberg — the Dynasty star and daughter of Princess Elizabeth of Yugoslavia — says her daughter India, 26, has been brainwashed.

She rejected claims that the group's alarming behaviour was consensual. "Ultimately, they're all victims of Keith Raniere," she says.

For years, ex-members and mental health experts have accused Nxivm of using textbook cult techniques such as sensory deprivation, brainwashing and isolating members from their friends and family.

Yet US law enforcement agencies always concluded that any behaviour was consensual.

But the authorities decided to act late last year when details emerged about the branding.

Now, as this lurid trial continues to reveal ever-more disturbing stories, the question remains: how on earth would any woman agree to be enslaved and branded?

Several ex-members have compared their predicament to a frog jumping into a pan of water.

A frog would jump out immediately if the water was hot — but heat it gradually and the frog won't realise its peril until it's too late.

The women were corrupted slowly, they say. Slowly but surely.