USA attorney Richard P. Donoghue said in a statement Friday that Mack recruited women for Raniere, and that the women were "exploited, both sexually and for their labor".
The #MeToo men are getting their comeback tours.
Mack allegedly recruited women into a sex trafficking pyramid scheme within Nxivm.
"Allison Mack recruited women to join what was purported to be a female mentorship group that was, in fact, created and led by Keith Raniere", Richard Donoghue, US Attorney for the Eastern District in NY, said in a statement.
Prosecutors say he had a "rotating group of 15 to 20 women with whom he maintains sexual relationships", with Mack said to have lured in many of his victims through an internal programme called The Source, which targeted actors to become his "slaves".
They allege that "slaves" in the group had to eat extremely low-calorie diets because of Mr Raniere's preference for thin women. She is accused of requiring members to "engage in sexual activity with Raniere" in return for "financial and other benefits" from him.
Mack and Raniere's arrests follow a report from the New York Times published last Fall with accounts of former members who said women were extorted into having sex and were branded with his initials on their hips.
A United States actress has been arrested and charged after she allegedly recruited women into a secret society and forced to have sex with a self-help guru.
But "the victims were then exploited, both sexually and for their labor to the defendants' benefit", said U.S. Attorney Richard P. Donoghue of the Eastern District of NY in Brooklyn. This blackmail was then used to both keep them in DOS, and maintain its secrecy.
Several aspiring and established Hollywood actresses have had varying levels of involvement with Nxivm.
Mack and co-star Kristen Kreuk, who claims she left Nxivm five yeas ago.
She is due in court later on Friday.
According to Page Six, Mack pled not guilty. Raniere is to be arraigned on his indictment later.
She ordered the slaves to have sex with Raniere and to pose naked for photos - "including on one occasion close-up pictures of their vaginas", the feds charge.
"Today we announce an additional arrest, and an indictment, in a case that brought to light an inconceivable crime", FBI Assistant Director-in-Charge Sweeney said.