how is montmorency revealed almost as the fourth member ?

In Chapter One, Montmorency is immediately noted as a member of the group. He is not only noted.... he's allowed an opinion. 

There were four of us—George, and William Samuel Harris, and myself, and Montmorency.  We were sitting in my room, smoking, and talking about how bad we were—bad from a medical point of view I mean, of course.

The only one who was not struck with the suggestion was Montmorency.  He never did care for the river, did Montmorency.