A careful reading of the text reveals tiny little clues about the Forest People, that John scatters throughout his narration. The first mention of them come as he describes his own people. He says that his people are "not ignorant like the Forest People" because they spin and sew their own clothes, and can read. The Forest People have become illiterate and can't read "the old writings" like the priests in his tribe can. He also mentions that the Forest People eat "grubs from the trees."
The next mention is that the Forest People are "afraid of the Dead Places," meaning, any place that has been nuked or destroyed by the "great burning" that was probably a nuclear holocaust of some sort. The Forest People stay away from those areas, and from the city of the Gods. He also mentions that the Forest People travel in hunting parties, and "could have killed" him if they came across him. So, they are a people that kill and fight often, hunting their game and killing those that do not belong in their tribes.
The overal impression that is gathered from these descriptions is that John and the Forest People are a lot like the original Native American tribes--they both have different ways of living and gathering food, and different traditions and cultures. It is that feel, in fact, that makes this story so intriguing, because at first glance, it is a story set in ancient tribes, but in reality it is in the future. I hope that those thoughts helped; good luck!