To join is to connect, attach, or become a member of. You could join two puzzle pieces together or join the Girl Scouts; it's your choice.
You can join two plastic tubes to make a long tunnel for your pet hamster, and you can also join a club — in other words, become a member or a part of the group. You can also join in with a song or join the staff of a swimming pool as a lifeguard. Join comes from the Old French word joindre, "connect or unite," from the Latin root iungere, "to join together, unite, or yoke."