In your opinion, why does Montag approach the campfire in the forest?

I think that there are two things that contribute to
making Montag approach the campfire after he has escaped across the
river.

First, he must surely be thinking that anyone who is
hanging out by a campfire out in the wilderness is a rebel like him.  We know that the
people in this society all live in the cities.  They do things like watching the parlour
walls and going to the amusement parks and driving way too fast in their "beetles." 
They do not sit around campfires out in the middle of
nowhere.

Second, as he watches the men, he can see and hear
that they are talking.  We know that people in the society don't just sit around and
talk.  Clarisse's family did and they were seen as really antisocial.  So these men are
doing something that "normal" people would never do.  Therefore, they must be rebels
like Montag.

For these two reasons, it is clear to Montag
that these are rebels like him and will probably want to help him.  That's why he
approaches them.