The Fellowship of Saint Giles is a group of people who have been infected by Red Court vampires, but have yet to drink the lifeblood of another human to complete their transformation.
The Fellowship helps its members control their urges, support each other, and fight the vampires who infected them.
They have mystic tattoos that are a binding, a spell cut into the skin to hold the darkness inside, grant them more restraint. And to warn them when it's rising, usually when they expend too much strength and are in danger of losing control. Normally the tattoos are not visible, but as the person uses the abilities their partial transformation gives them they begin to change. They initially are black and seem to swirl on the skin. As the Fellowship member reaches the danger zone, the tattoos turn red.
In Changes, they're described as a collection of the supernatural world's misfits and outcasts, many of them being half-vampires. They hate the Red Court with a passion and do everything they can to confound them. They operate in cells, chose targets, train recruits, plant bombs—basically like terrorists. They fund their operations through many shady business activities. 
In The Series Edit
In Death Masks, it's revealed that Martin and Susan Rodriguez came to Chicago because they knew that Paolo Ortega was intending to kill Harry and if successfull, the War would end peacefully to begin again in twenty years when the Reds were much stronger. Martin says he was sent to make sure that Ortega did not kill Harry Dresden and to eliminate Ortega if possible. Ortega survived, his vassals took him off the field.
In Proven Guilty, the Fellowship is one of the Council's primary allies in The War with the Red Court. The Fellowship was turning out to be a formidable ally having been touched by the Red Court's dark powers and with a real interest in seeing it's downfall. They are not capable of challenging the Reds in face-to-face combat, but wreak havoc on their suppy chains, intelligence and support from the mortal side. They help provide the Council with information on the Reds which has bought them needed time to launch successful raids and to regroup.
In Changes, they send Martin and Susan Rodriguez to locate the Reds who have taken Susan and Harry's child. Their first operation is to break in to the management office in Harry's building to possibly gain information on their plans and location.