(CNN) The members of the upstate New York group called Nxivm looked up to Keith Raniere as a leader who spread his ideas on how to achieve success and happiness but, prosecutors say, he was actually a conman who took advantage of vulnerable people.

"He targeted people who were looking to improve their lives," said Assistant US Attorney Tanya Hajjar in her opening statement Tuesday at Raniere's trial in New York City. "Once he gained their trust he took advantage of them. The defendant said that he was a mentor, but he was a predator."

Raniere, 58, has been charged with racketeering, sex trafficking, sexual exploitation of children, and human trafficking, among other offenses. He has been held in federal custody in Brooklyn since his arrest in Mexico in March 2018. On Tuesday, he appeared in court wearing a gray sweater with a button down shirt, seated next to his attorney Marc Agnifilo.

"I certainly agree with one thing," Agnifilo said in his own opening statement, in response to claims raised by prosecution. "You are going to hear the truth."

Agnifilo argued that the prosecution's own witnesses will testify that they loved being part of Nxivm -- an organization that prosecutors have deemed a pyramid scheme but others call a cult.

Nearly 17,000 people took the self-help classes that Nxivm offered, Agnifilo said, and he urged the 12 jurors to look at the facts of the case through Raniere's perspective.

Raniere's five other co-defendants have pleaded guilty and might testify against their former mentor. Seagram liquor heiress Clare Bronfman, "Smallville" actress Allison Mack, Kathy Russell, Lauren Salzman and Nancy Salzman have pleaded guilty to crimes ranging from visa fraud to racketeering.

One former Nxivm member, identified only as Sylvie to protect her identity, began testifying Tuesday afternoon about coercion she said she experienced while part of the organization.

Raniere has pleaded not guilty to all charges. If convicted, he could be sentenced to life in prison.

Recruits provided 'collateral' information, prosecutors say

Keith Raniere, 58, at a hearing at federal court in Brooklyn in March 2018.

Nxivm, created by Raniere in 2003, exploited recruits "both sexually and for their labor, to the defendants' benefit," said US Attorney Richard DonoghUe.

An affidavit prepared by an FBI agent that was filed in support of the arrest warrant said Nxivm (pronounced NEX-ium) was based in upstate New York and operated like a multilevel marketing scheme, pushing members to take classes that cost as much as $5,000.

It encouraged members to take more classes to move up the ranks and to recruit other members to help create more revenue, which left some members in debt to Nxivm itself, the affidavit said.

The members in Nxivm called themselves "Nxians," and wore different colored sashes that signified their ranks, the affidavit said. They signed nondisclosure agreements and promised not to reveal certain things about Nxivm's teachings, the affidavit said.

Within the group was a secret society called DOS that prosecutors say consisted of dozens of women who were "masters" or "slaves." The affidavit said DOS masters went after recruits who were "experiencing difficulties" in their lives.

While recruiting them, the masters would insist the recruits provide information about themselves as "collateral," to encourage the recruits to keep information about DOS a secret, the affidavit said.

The collateral included sexually explicit photos, videos in which the member tells damaging stories about themselves and letters making damaging statements against friends or family, the affidavit said.

DOS had a pyramid structure, with Raniere at the top as the highest master and the only male, the affidavit said.

Women who served as "slaves" were expected to complete tasks for their masters, and could be punished if they failed, the affidavit said. The affidavit said some slaves were expected to have sex with Raniere and stick to extremely low-calorie diets to stay thin -- because Rainere liked thin women.

In one of the most bizarre allegations, some DOS members were branded with a cauterizing pen on their pubic regions in ceremonies that were recorded and could take half an hour, the affidavit said.

"Some DOS victims were told that the brand stood for the four elements (the lines represented air, earth, and water and the cauterizing pen represented sealing with fire)," the affidavit said. "Based on information obtained during the course of the investigation, however, it is clear that the brand in fact consisted of Rainere's initials."

The man at the top

Raniere's mission was teaching others about how to achieve true happiness, said his lawyer, Agnifilo.

"Part of what I feel I have to prove is that Keith Raniere is not a monster," Agnifilo said. "He's a well-intentioned person who spent a great deal of time trying to figure out how to help people be happier."

Raniere denies Nxivm is a cult, Agnifilo said.

"He's nervous that the case has created a lot of what he views as sort of prejudice against him and his ideas," Angifilo said. "He firmly believes his ideas are sound ideas, are good ideas, are humanitarian ideas.

"He thinks DOS is a good idea and is a pro-woman group. He created it to have women have their own society ... where men would play no role."

Before starting Nxivm, Raniere had founded a company called Executive Success Programs which was supposed to "actualize human potential," said the affidavit.

Is Nxivm really a cult?

Prosecutors don't go as far as to call the group a cult in court filings.

But cult expert and former cult member Janja Lalich -- who was not a member of Nxivm and is not a witness in the trial -- says it "absolutely" is a cult, which she defines as a group or social movement with a shared devotion to a charismatic, authoritarian leader that's structured in such a way that it asserts influence and control over its members to keep them under their control. She believes this is the biggest trial of a purported cult leader in nearly a decade.

"The Nxivm courses were used as a way to draw people into the more restricted and more perverse inner circles, including DOS," Lalich said. "In a sense, Nxivm is a front to draw people in to be able to target certain people and pull them in further and further."

Lalich said that most people who become involved with cults were recruited as members by family or friends. And once inside, it became difficult to leave.

Actress Catherine Oxenberg, a former star on the TV show "Dynasty," also thinks Nxivm is a cult. Oxenberg was able to get her daughter, India, out of the group. She had no comment on if India was asked to testify in the trial.

Mother and daughter started taking Nxivm classes in 2011, thinking they would learn about entrepreneurship, Catherine Oxenberg said in an email to CNN. India became more engrossed in the classes and moved from Los Angeles to Nxivm's upstate New York headquarters, her mother said.

"One of the most deeply evil tricks of cults, Nxivm/DOS in particular, is that they hide behind the veneer of choice, even choices many rightly say are virtuous," Catherine Oxenberg said. "But in cults such 'choices' only serve the perverted goals of the leader."

Testifying could be 'empowering but devastating'

Bronfman, Mack, Russell, Lauren Salzman and Nancy Salzman were accused of serious crimes of their own, including prosecutors' allegation that they "were aware of and facilitated Raniere's sexual relationship with two underage victims," including a 15-year-old girl. Dozens of child pornography images are part of the evidence.

Allison Mack leaves the Brooklyn Federal Courthouse on February 06.

Mack, 36, pleaded guilty in April to racketeering and racketeering conspiracy charges and faces up to 40 years in prison. She was a DOS master and helped co-found DOS, which was targeted toward actors, prosecutors said.

Bronfman, who served on Nxivm's executive board for nearly a decade,used her wealth to help finance Nxivm's activities, prosecutors said. She tearfully pleaded guilty in April to concealing and harboring undocumented immigrants for financial gain and fraudulent use of identification. She faces up to 27 months in prison and agreed to pay $6 million in forfeiture to the government.

Lalich said she has spoken with about a dozen former Nxivm members, and believes it will be difficult for Raniere's former followers to face him in court, especially those who have recently pleaded guilty.

"It'll feel empowering but it'll also be devastating," Lalich said. "I'm hoping that the attorneys have a good support system set up for all of those women."

Agnifilo told CNN he believes that if any of the women who have pleaded guilty testify against Raniere, it would only help his case, and that Raniere has repeatedly told his attorney that he hopes his co-defendants don't have to serve prison time.

"I think they're going to be important witnesses for both sides," Agnifilo said. "I think they're going to give an accurate, balanced perspective."