An heiress to a Canadian alcohol fortune has pleaded guilty in a sensational American case, accusing a cult-like upstate New York group of creating a secret harem of sex slaves for the group's self-anointed spiritual leader.

Key points: Clare Bronfman pleaded guilty to fraud and harbouring someone in the US without pay

Clare Bronfman pleaded guilty to fraud and harbouring someone in the US without pay She bankrolled various efforts of NXIVM, a self-styled self-improvement program

She bankrolled various efforts of NXIVM, a self-styled self-improvement program Its leader has been accused of creating a secret society of women who he then had non-consensual sex with

Clare Bronfman admitted in her plea in federal court in Brooklyn that she harboured someone who was living in the US illegally for unpaid "labour and services", and that she committed credit card fraud on behalf of Keith Raniere, the lead of a group called NXIVM (pronounced "nexium") that promised its followers self-improvement.

Bronfman is the 40-year-old daughter of the late billionaire philanthropist and former Seagram chairman Edgar Bronfman Senior, who moved the family company's fortunes from alcohol and sprits into a disastrous bid for entertainment companies in the late-20th century.

The move eventually led to the sale of the Seagram company in 2000.

As part of a plea agreement, Bronfman agreed to forfeit $US6 million ($8.4 million) from a fortune prosecutors say is worth $US200 million ($279.4 million), and will face more than two years in prison at sentencing on July 25.

A 2010 Vanity Fair profile of Bronfman's relationship with NXIVM detailed years of financial support of the group from the Bronfman coffers.

"According to legal filings and public documents, in the last six years as much as $US150 million was taken out of the Bronfmans' trusts and bank accounts, including $US66 million allegedly used to cover Mr Raniere's failed bets in the commodities market, $US30 million to buy real estate in Los Angeles and around Albany, $US11 million for a 22-seat, two-engine Canadair CL-600 jet, and millions more to support a barrage of lawsuits across the country against NXIVM's enemies," the profile wrote.

She told the judge that she had wanted to help people through NXIVM but ended up dishonouring her family.

"Your honour, I was afforded a great gift by my grandfather and father," Bronfman said.

"With the gift, comes immense privilege and more importantly, tremendous responsibility. It does not come with an ability to break the law.

"For this, I am truly sorry."

Bronfman's father believed NXIVM was a cult

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The plea means Bronfman will avoid going to trial early next month with Mr Raniere, who is facing conspiracy charges that allege his inner circle of loyalists created a secret society of women who were forced to have non-consensual sex with him.

In an interview with Forbes in 2003, Bronfman's father said he thought NXIVM was a "cult" and expressed concern about his children's "emotional and financial investment" in Mr Raniere's program.

Bronfman's older sister, Sara, was also a participant in NXIVM.

Prosecutors say some of the women were branded with his initials as part of their initiation.

An accountant for the group, Kathy Russell, also pleaded guilty on Friday to a fraud charge.

She joined three other NXIVM insiders besides Bronfman who have also pleaded guilty.

Among those defendants is Allison Mack, the TV actress best known for her role in US television series Smallville.

Bronfman had long been affiliated with NXIVM, giving away tens of million of dollars to bankroll Mr Raniere and his program of intense self-improvement classes.

She also paid for lawyers to defend the group against a lawsuit brought by its critics.

With the two guilty pleas on Friday, Mr Raniere will face a jury by himself.

Defence attorneys have insisted any relationship between their client and the alleged victims, including women expected to testify against him at trial, was consensual.

ABC/AP