Note that the clubs have all kinds of rules. Some are OFTR members, some aren't. Some are open to any riders and some aren't. Some are stewards of local forests and some have private land and use agreements. Some are family trail riding oriented and some are competition oriented etc
Why should you join a club? In many forests, membership is required. In either case, the local club is formed by people with the same interests, at least as far as trail riding goes. Together, they know more places to ride and more things about dirt bikes than you! There's always some organized rides etc.
Which club to join? Probably the one that covers the area where you'll do most riding will give you the most value, special interests aside.
These are the off-road clubs: