When Montag rejects the censorship values of society, he is cast out and hunted by the mechanical hound for sport, broadcast on television. When fleeing, he meets Granger, who gives him a drink to change his biochemistry so the hound cannot track him anymore. Once they are safe, Granger shows Montag the hidden society of people who memorize books:
"...We all made the right kind of mistakes, or we wouldn't be here. When we were separate individuals, all we had was rage. I struck a fireman when hecame to burn my library years ago."
(Bradbury, Fahrenheit 451, Google Books)
Granger's group is something that Montag has been searching for without realizing it. After finding Granger, Montag's life gains a new purpose, to create and protect books instead of destroying them. Granger's influence gives Montag the ambition to continue reading and learning, instead of running blindly without a goal.
You have to keep in mind the authors perceptions. Like you know, Granger is the hidden name for people which informed the "Stasi" in the Ex-DDR. Now we know, what he really represent in this context.
Granger stand on one level with Beatty, maybe they could be the same persons. He is not the man that he seems to be. He is a little bit like Montags "bad side". He is a bad person, have to live in the wood andtries to trick Montag.
To perception of the book people, Montag realize that the book people are his last chance to change the world to ideal world, who everybody can live free without police and the firemen. But when he notice that the book people are not tough enought to fight with him in the revolution his dream burst. Afte Granger start to controll Montag with his mental force, Montag become more quiet and accommodate to the other book people. In the end he really relish the living together with the other book people.
Granger symbolize the devil and he has got a bad influence on Montag. The affect is, Montag lose his values and norms and become a conditioned marionette who do what ever Granger want him to do. Montag revelop his thinking and appears like in the beginning.