I have had great experiences with book groups, as well as some experiences that were not so great. There are many benefits from a book group experience, and I think that most people can find a group they enjoy. Finding an established book group can be easy, but discovering if it is a good match for you may not be as easy.
Here are a few ideas for evaluating a potential book group. Realize that every group has a different focus and style. Some groups focus much more on the social aspect of the group. They are about the meeting, not the reading. I was once in a group like this, and I enjoyed it well enough for a while. However, discussions began to be derailed because a couple people would not finish the book and would insist that we not discuss the endings at the group. You may be fine with this policy, but it was quite frustrating for me.
Some groups are directed by one person. Others are a shared responsibility. I have been in good groups with both styles. Some groups are held at libraries or book stores, while others are held in the homes of members. I know of a few groups that meet at restaurants. There are pros and cons for all locations, but these are much less important than the camaraderie and discussion.
Pay attention to how the group interacts with one another. The social aspect of the group will determine many other factors, including whether a group will last. Are the discussions intense? That is fine as long as there is an underlying current of respect. In fact, it is probably a mark of a good group if the members do not shy away from difficult subjects. I have been in one book group that discussions were continually shut down by one member of the group who had a particularly condescending way of disagreeing.
Book selection and discussion are the two largest factors you should evaluate when looking at a book group. Some groups do not have a stated book list, but allow members to make suggestions. Although these groups could be considered eclectic, they tend to read mainly contemporary fiction. Some groups have a more formal book list to ensure a greater variety of books. It may be possible to know a year or more in advance what books will be read. Some groups have a genre that they read, such as mysteries or science fiction or religious books. Taking a look at a book list for the next few months can give you a real feel for whether a book group would work for you.
The discussions when the group gets together are the main part of the book group experience. Some groups discuss literary techniques, the author's life, and plot structure. Other groups discuss only the themes from the book. Some groups discuss how the book relates to their own lives, and how the book changed them. Your own preference should be the deciding factor on whether you join a particular book group or not. Discussions are best when all members have a common goal, and respect for each other.
When you join a book group, you should select it carefully. It can be a great experience if it is a good match for you.