Hand Signals
There is an unspoken code of respect between motorcyclists. There is a specific way that members of biker clubs wave to each other, by extending their hand low instead of high. They also use specific hand signals to warn one another about speed traps, police, or other travel-impeding problems that may lay ahead. Most motorcycle gangs have their own group signals, which they use to communicate with rival gang members to tell them that they mean no harm.
Lots of Veterans
The founding members of Hells Angels were veterans of WWII. The name of the club was inspired by the Flying Tigers' "Hells Angels" squadron. This helped lots of veterans find like-minded individuals after getting out of wars, although the Hells Angels are generally considered to be bad people.