Smallville actress Allison Mack pleaded guilty in a New York court Monday to racketeering charges related to her role in a cult-like group whose leader has been accused of sex-trafficking.

Mack entered her plea as jury selection was about to begin in the trial of her co-accused, Keith Raniere, the founder of NXIVM, a self-improvement organization that prosecutors have likened to a pyramid scheme.

Mack wept as she admitted manipulating women into becoming sex slaves for Raniere, known to followers as "Vanguard."

"I believed Keith Raniere's intentions were to help people, and I was wrong," Mack told a federal court judge in Brooklyn.

Mack was charged with conspiracy to commit sex trafficking and conspiracy to commit forced labour.

She was part of a secretive inner circle of female NXIVM members described by prosecutors as being "slaves" to Raniere.

Members of NXIVM's inner circle claimed they were branded with a scar that prosecutors claim contained the initials of leader Keith Raniere. (Supplied by Sarah Edmondson)

As part of their initiation, the members of a group — known as DOS — allegedly gave "collateral" in the form of damaging secrets that could be released should they fail to comply with the wishes of the leaders of the group.

The women in DOS were also allegedly branded in the pubic region during super-secretive ceremonies with a symbol that later appeared to be Raniere's initials.

At its height, NXIVM had thousands of members in New York, Vancouver and Los Angeles. Mack was a prominent member of the group during the time the TV series Smallville was filmed in and around Vancouver. Mack played the role of Superman's close friend.

She also became a prominent adherent of NXIVM, appearing in many videos alongside Raniere as he explained his philosophy.

According to documents filed with the New York court last month, Mack was "engaged in negotiations with the government regarding a potential pretrial resolution of her case."

"I know I can and will be a better person," Mack told the court.

Her sentencing was set for Sept. 11.

Along with Raniere, Seagram's heiress Clare Bronfman still faces racketeering charges related to the operation of NXIVM.

The trial for NXIVM leader Keith Raniere was supposed to get underway with jury selection on the morning that his co-accused Allison Mack entered her guilty plea. (Elizabeth Williams via AP)

Bronfman is also accused of conspiracy to commit identity theft, encouraging and inducing illegal entry to the United States and money laundering. She has pleaded not guilty to the charges and was released last July on $100-million bail.

In a statement posted to her website in December 2017, Bronfman said, "There have been many defamatory accusations made and I have taken them seriously. Determining the truth is extremely important to me, and I can say firmly that neither NXIVM nor Keith have abused or coerced anyone."

Mack is the third person to enter a guilty plea in the case. Former NXIVM executive Nancy Salzman and her daughter, Lauren, have both entered pleas.

Raniere, who has been in custody since 2018, has pleaded not guilty to the charges against him.