If you want to join a Crew, you can find Active Crews with open membership at the Rockstar Games Social Club website – make new Friends and allies and then take to the streets to battle with other Crews.
You can improve your reputation and earn cash by completing Jobs in a Crew. If you rank your Crew up, you will unlock special bonuses. Or you and your Friends can create a private Crew and really go after strangers. If you don’t want to join a Crew, then have fun playing with yourself.