The Preachers were an group of people originating in Pete's World that were finding out about the true intentions behind CEO John Lumic of Cybus Industries, and his creation: the Cybermen.
Distinguishing signs and equipment
Before John Lumic's downfall, the Preachers travelled in a blue van. After the downfall of John Lumic when Jake and Mickey hired more men to help increase the strength in numbers, the Preachers had a drastic change: instead of improvising clothing, they now wore black paramilitary coveralls and an oxygen mask. Their emblem looked like a circled P. (WC: Tardisode 5, TV: Rise of the Cybermen) When they travelled to the Tenth Doctor's universe, they also carried large energy weapons. They also possessed large dimensional transporters from Parallel Torchwood which allowed them to cross between their universe and the Doctor's. (TV: Doomsday)
History
The London cell of the Preachers included their leader Ricky Smith, Jake Simmonds and Mrs Moore. (TV: Rise of the Cybermen) Pete Tyler, under the codename Gemini, passed information to them covertly - an accident on his part, as he thought he was passing information to the security services. After Ricky died, his counterpart from the Tenth Doctor's universe, Mickey Smith, took his place. Mrs Moore had also died in the meantime. (TV: The Age of Steel)
When they found out what the Pete's World version of the Torchwood Institute were doing, the People's Republic had the institute shut down. Mickey Smith was able to cross back through the Void to his home universe using one of the medallions developed by Torchwood. He infiltrated Torchwood 1 disguised as a lab technician, and investigated the Void Ship. Mickey incorrectly believed it contained the ultimate Cyberman leader, such as a CyberKing. The Void Ship actually contained the Cult of Skaro. (TV: Army of Ghosts) Jake Simmonds and Pete Tyler soon followed Mickey from their own universe. The Preachers aided the Doctor in his battle with the Cybermen and Daleks in his universe. In the end, the Preachers returned to their world while the Doctor and Rose Tyler banished the Cybermen and Daleks into the Void. (TV: Doomsday)
Arthur King and Charlie Haughton were commanders of the Preachers and helped them to destroy the Cybus factories across the world, sending in soldiers to disable the emotional inhibitors. Jake Simmonds supported King and Haughton by sending them messages. (GAME: Cyber Assault)
Harriet Jones combined the Preachers with Torchwood in order to create UNIT. (AUDIO: The Siege of Big Ben)
Behind the scenes
The Preachers' emblem, a circled "P", was seen in the Doctor's universe. It was seen as faded graffiti on rusty bins outside Bernie's flat in Cardiff (TV: Ghost Machine) and inside the Ritz. (TV: Captain Jack Harkness)