A club is a group of people who share an interest, whether it's playing chess or hiding out in a tree house together.
You might join a club if you love to play golf or discuss books with a friendly bunch of people. This kind of club is a formal group — another type is simply a big, heavy stick, or a long metal one used for hitting a golf ball. You can even use it as a verb: "Don't club that snake! Just move it over to the field." The root word is the Old Norse klubba, or "cudgel, club."