An actress from the television series “Smallville” was charged on Friday with sex trafficking for her involvement in an organization promoting itself as a self-help group for women that forced its members to have sex with its leader, federal prosecutors said.

The actress, Allison Mack, 35, helped gain recruits for what was purported to be a mentorship group but instead exploited its followers, who were branded with a symbol containing the initials of the organization’s leader, said Richard P. Donoghue, the United States attorney for the Eastern District of New York, in a statement.

Ms. Mack participated in a group know as Nxivm (pronounced Nex-e-um), the authorities said. The organization, near Albany, was led by Keith Raniere, 57, who was arrested in March by federal officials in Mexico on sex-trafficking charges. Its followers were forced to have sex with him because they feared that if they did not do so, compromising material that they had provided about themselves would be released publicly, according to a complaint.

The group has denied it was a cult, but former members have said that Mr. Raniere demanded obedience from his followers, who referred to him as “Vanguard.”

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Mr. Raniere created a secret society within Nxivm called “DOS,” which derived from a Latin phrase that loosely translated to “Lord/Master of the Obedient Female Companions,” or “The Vow,” officials said in the statement.

“DOS operated with levels of women ‘slaves’ headed by ‘masters,’” the statement said. “Slaves were expected to recruit slaves of their own (thus becoming masters themselves), who in turn owed service not only to their own masters but also to masters above them in the DOS pyramid.”

Ms. Mack was one of the women on the level of the pyramid immediately below Mr. Raniere, officials said.

The slaves were often required to perform “acts of care” and pay “tribute” to their masters, including doing work and other tasks for which they were not compensated, according to a letter federal prosecutors filed with the court.
They also had to participate in “readiness drills, which required them to respond to their masters any time of day or night, causing DOS slaves to be seriously sleep-deprived,” the letter said. The women, it added, were also branded in their pelvic regions with a cauterizing pen “with a symbol that, unbeknown to them, incorporated Raniere’s initials.”

Ms. Mack was arraigned on Friday afternoon and a plea of not guilty was entered on her behalf. She was held without bail, with a bail hearing set for Monday.

A publicist for Ms. Mack declined to comment on Friday evening. Lawyers representing her could not immediately be reached on Friday evening.

Ms. Mack played Chloe Sullivan in “Smallville,” which told the story of the upbringing of Clark Kent in Smallville, Kan., before he became known as Superman. In the series, which lasted for 10 seasons starting in 2001, Ms. Mack’s character was one of his best friends. Ms. Mack is also known for her roles in the movies “The Ant Bully” (2006) and “Honey, We Shrunk Ourselves!” (1997).

Another actress on “Smallville,” Kristin Kreuk, wrote on Twitter last month that she took an intensive personal growth course with Nxivm to help overcome her shyness, that she left five years ago and that during her involvement she never experienced any “illegal or nefarious activity.”

“I am horrified and disgusted by what has come out about DOS,” she wrote.

Ms. Mack’s biography on her website, which is no longer active but is accessible in internet archives, said that after “Smallville” ended, she immersed herself in the study of acting in “both conventional and unconventional ways.”

The more unconventional approach happened when she came across the work of Mr. Raniere, the website said, adding: “Over the course of several years, Mr. Raniere mentored Allison in her study of acting and music. As such, she has developed a deep connection to the nature of humanity as it relates to acting as an art form, and a tool for personal evolution.”