The Hellfire Club was group of men in the 18th century that were in the occult. Since they sided with the British against American independence, Benjamin Franklin infiltrated the Hellfire Club as a spy, and obtained the key to Purgatory.
The Hellfire Club was hired by War to capture Katrina Crane, to use her as a vessel to bring Moloch into the world. Abbie Mills told Sheriff Reyes about them, so they could have a tactical team to raid their warehouse.
Abbie guessed their "666" password correctly, unlike Crane who thought it may be other numbers significant in the club's history.