A television actress best known for playing a young Superman's close friend pleaded not guilty Friday to sex trafficking after federal prosecutors said she worked as a slave "master" recruiting unsuspecting women to a cult-like group led by a man who sold himself as a self-improvement guru to the stars.

Actress Allison Mack recruited slaves into a pyramid scheme for the benefit of alleged sex cult leader Keith Raniere, federal prosecutors said in an indictment unsealed against her Friday. 

Mack, 35, starred in the CW series "Smallville" and has played minor roles since the series ended in 2011. But authorities said she was a major player in Raniere's cult-like organziation called NXIVM (pronounced Nex-i-um),  helping to recruit unsuspecting women for what was purported to be a female mentoring group. 

But "the victims were then exploited, both sexually and for their labor," according to federal prosecutors.

"Mack and other ... masters recruited ... slaves by telling them that they were joining a women-only organization that would empower them and eradicate purported weaknesses the NVIVM curriculum taught were common in women," prosecutors said.

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Prosecutors said Friday that Mack stood at the level directly below Raniere in a pyramid scheme called DOS, into which she recruited fresh slaves. The government alleges that Mack forced those slaves to have sex with Raniere in exchange for what were described as "financial and other benefits."

Prosecutors said she required women she recruited to engage in sexual activity with Raniere, who paid Mack in return.

Assistant U.S. Attorney Moira Kim Penza said in court that that "under the guise of female empowerment" Mack "starved women until they fit her co-defendant's sexual ideal."

In a letter attributed to Raniere  -- who was arrested in Mexico and returned to the United States last month --  previously posted on a website related to NXIVM, he denied the practices were sanctioned by the self-described self-help group.

 The complaint against Raniere - known in the group as "Vanguard" - said that many victims participated in videotaped ceremonies where they were branded in their pelvic area with a symbol featuring Raniere's initials.

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"During the branding ceremonies, slaves were required to be fully naked, and the master would order one slave to film while the other held down the slave being branded," the complaint says.

Raniere left the United States late last year after The New York Times reported the stories of some women who defected from their secret sorority and the government began interviewing potential witnesses. He sought to cover this trail by using encrypted email and ditching his phone, court papers say.

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Both Raniere and Mack face charges of sex trafficking, sex trafficking conspiracy and forced labor conspiracy, for which they could receive 15 years to life in prison. 

Mack's "Smallville" co-star Kristin Kreuk says she was involved with one of the group's self-help programs but left about five years ago. She wrote on Twitter last month that she didn't experience any "nefarious activity" and was "horrified and disgusted" by the allegations.

She entered her plea and was remanded to custody after Judge Cheryl Pollak refused a request from Mack's lawyers to release her without bail. A bail hearing will be held Monday.