Some good advice from Richard Ridley and CreateSpace: CreateSpace Community: Resources: Participate in Online Book Clubs.
Notice Richard doesn’t suggest you join these groups just to market your work. Being a part of a group that actually fits you is helpful to make connections and share information. You might, at some time, share your book(s), but if you go into groups like these with the sole purpose of marketing, then you will probably be disappointed. Participation is a good marketing strategy, but it takes time and effort – ugh!