Remus became friends with James Potter, Sirius Black and Peter Pettigrew at the Hogwarts Express on his first trip to Hogwarts. 

He was sleeping in the train since last week he had undergone his transformation and was quite worn out. However halfway through the journey, he was woken up by hearing a commotion. The boy sitting next to him, Peter, was staring at the door in fright out of which noises were coming. 

Shortly afterwards the Head Girl entered holding two boys by the collar. One had tousled hair and spectacles while the other had shoulder length hair that would have been neatly combed before but some strands were standing up as if electrocuted. James and Sirius.

The Head Girl was scolding them for trying to escape from the train and threatened them to sit still in the compartment or else she would complain to the Headmaster.

After she had gone and Remus was fully awake and curious on why they had pulled such a stunt, both of them introduced themselves and that was the start of a good long friendship for the four boys.