If you wanted to see sexually explicit pictures of children, you had to be willing to share.

According to court filings, that was the main security measure taken to block undercover cops from discovering a members-only, online child pornography network now being targeted by federal prosecutors in Pittsburgh.

Stephen Sims, 55, of San Leandro, Calif., is accused of acting as the gatekeeper for the network, requiring new recruits to send him images of naked children before he allowed them to become full members.

Mr. Sims and seven other people have been arrested and charged in connection with the case, acting U.S. Attorney Robert S. Cessar announced yesterday. As many as 100 people have been connected to the network.

"The ongoing investigation has both a nationwide and international reach," Mr. Cessar said.

The investigation began in February 2008, when officials arrested an unidentified Western Pennsylvania man who agreed to tell them about the pornography network, according to a criminal complaint for Mr. Sims.

The man identified Mr. Sims as a "security officer" for a group operating on Multiply.com, a Web site that allows users to share pictures and videos.

"[Mr. Sims] vets potential new group members and advises current members regarding group membership and security practices that are intended to hide the group's illicit activities from discovery," the complaint said.

He called himself the "den mother" of a "completely private group" of "like-minded individuals."

Mr. Sims would require recruits to send him explicit images to prove that they had access to child pornography and weren't law enforcement officials. He also told them that their Multiply.com accounts should be set so only other group members could see postings.

With the help of a second cooperating witness, Special Agent James Kilpatrick of the Pittsburgh office of U.S. Immigration and Customs Enforcement was able to identify more than 100 potential participants in Mr. Sims' group.

In February 2009, Multiply.com complied with a federal search warrant and gave Agent Kilpatrick access to a hard drive that contained the content of the accounts, including many graphic images of children engaged in sexual acts, both with adults and other children.

Multiply.com also released records connecting Mr. Sims to the site.

"This type of content is disturbing and illegal, and has no place on sites where users generate content, which is why we've worked with authorities and people have been arrested, and will hopefully be going to jail," Marc Bernstein, a spokesman for Multiply, said in an e-mail message. "This case is still pending and we continue to work with authorities."

Mr. Sims was apprehended in September and charged with child exploitation and conspiracy to distribute and receive child pornography. At the time he was an employee of the Del Monte Meat Co. in Oakland, Calif.

Also arrested were Fred Woolum, 58, of Lexington, Va.; Patrick Carney, 65, of Manchester, N.H.; Paul Schwartz, 48, of Lathrop, Calif.; Daniel Cox, 53, of Houston, Texas; Brandon Miller, 22, of Omaha, Neb.; Ryan Chiles, 21, of Hampton, Va.; and Dave Dean, 42, of Houston. Mr. Carney appeared in U.S. District Court here yesterday to plead not guilty to child exploitation and conspiracy charges, according to his attorney, Patrick J. Thomassey.

According to the U.S. attorney's office, the group participated in a "child exploitation enterprise" from Jan. 1, 2007 to Sept. 22 of this year. Six defendants are being held, while Mr. Schwartz and Mr. Miller have been released on bond.

The charges carry a sentence of at least 20 years in prison and a fine of $250,000.

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