With the caveat that they weren't exactly Satanists, there are two examples.

The first is the Affair of the Poisons in 17th century France. This was an incident where several high-ranking aristocrats were accused of fooling around with alchemy, fortune telling, love potions, black masses, and poisons. Though not everything people were accused of in the Affair of the Poisons was true, parts of it were. Namely, it seems true that Étienne Guibourg and La Voisin practiced Black Masses for the benefit of aristocrats and sometimes sacrificed babies. The babies they used were mostly the unwanted offspring of prostitutes and the like.

However, none of those involved seem to have been Devil worshipers. Instead, most of their fooling around with black magic seems to have been more in the tradition of goetia - that is, maintaining Christian belief while begging demons for favors. It's hard to say how many people actually were involved in the Affair of the Poisons, but somewhere around 30 people were executed for taking part of it. One of Louis XIV's mistresses, Françoise Athénaïs de Rochechouart de Mortemart, was implicated, but she was never formally charged and historians on whether she actually was involved or not.

The second is the strange case of Adolfo de Jesús Constanzo. Constanzo practiced a mixture of Palo Mayombe, Santería, and hoodoo and ran a business in Mexico City selling charms and ritual animal sacrifice. A lot of his clients were drug lords, corrupt cops, and hitmen. At some point in the 1980s, he became exclusively involved with the mid-size Hernandez drug cartel and started ritually sacrificing members of rival drug cartels to ensure the success of their drug shipments. There were approximately 27 people involved in this group at some point. In 1989, Constanzo and his followers made two fatal mistakes. First, they kidnapped and sacrificed an American college student named Mark Kilroy, whose disappearance set off a large-scale manhunt. Second, one particularly stupid member of the cartel/cult took Constanzo's promise of invisibility literally and routinely drove pass police checkpoints. That meant that the Mexican police followed the dimwitted boy back to the ranch that served as the cartel/cult's base.

Again, though, Constanzo and his followers were not Satanists. Palo Mayombe and Santería are both syncretic religions that blend the Yoruba religion with Roman Catholicism and human sacrifice is not normally involved in either of them.

I tend to be of the belief that the size of both of these groups indicate that it's mostly likely not possible to secretly operate a larger-scale secret human sacrifice cult. Even at the scale of both, they still got caught.

Sources:
• The Affair of the Poisons: Louis XIV, Madame de Montespan, and one of History's great Unsolved Mysteries by Frances Mossiker
• The Affair of the Poisons: Murder, Infanticide, and Satanism at the Court of Louis XIV by Anne Somerset  • Buried Secrets: A True Story of Serial Murder by Edward Humes
• Cauldron of Blood: The Matamoros Cult Killings by Jim Schutze