The Hell's Angels is one of the biggest motorcycle clubs in the United States, along with the Bandidos. The Blue Knights International is one of the largest public service-oriented clubs, with over 19,000 members, as of 2015.
Many law-abiding motorcycle clubs focus on local events, specific sports and interests, so they tend to have smaller memberships. This means that some of the largest clubs are outlaw gangs, which attract members because they can make money with illegal activity.
There are also large organizations that may not meet the typical definition of a motorcycle club, such as the American Motorcyclist Association. These organizations generally have a broader interest in advocating for rights and protections for bikers.