Judging by the hostility of Adam toward the book that they were discussing, this particular book club had nothing to do with the actual regular book club. Ben would have known when the date of the book club was, so he would have been there, since he read the book. This was a group of people that Juliet was trying to get to over throw Ben, since it was the day after she realized that her sister was alive and Ben was never going to let her leave.