We attend one at the local library when I'm in Tucson. The way that one works is that we all have read the book and then we sit around whole a leader will throw out questions about the book for discussion.
HERE'S a webpage from the University of Sheffield's book group with questions for discussion on Colour of Magic for example.
As you can see, the questions are quite vague and designed to stimulate discussion. When everyone contributes it can be quite interesting.
In the Tucson group we got them to read Going Postal and discuss it. Most of them had never even heard of Terry and certainly never read Fantasy. It was surprising how many of them enjoyed the experience, which was enhanced after we discussed the book.
Let us know how you get on. And maybe you'd consider going to the group again only with a book you are unfamiliar with. It's a great way to discover new authors.
Wyrd bið ful āræd.
"Coffee. The finest organic suspension ever devised. I beat the Borg with it."
- Capt. Kathryn Janeway