Well, Crabbe and Goyle are in Harry's 'circle' as the muscle I imagine. His social group really consists of Draco, Crabbe, Goyle, Daphne, Tracey, and Theodore Nott. As for the train, I imagine that Harry was chosen as prefect and so was Hermione. So we can have that meeting there? Perhaps Hermione is told to get Harry for the meeting or vice versa?