Don’t question their spelling. Hells Angels, that should actually be spelled differently. Still, you don’t have to point out to them that it should actualy be Hell’s Angels. After all, the club has its own rules, and language seems (not) to be part of it. The website even states that they are aware of this. “Yes, we know there’s an apostrophe missing, but it’s you who misses it. We don’t.”
There is a dress code. As a member of the motorcycle club you have to adhere to a certain dress code: you always wear a vest with the name and insignia of the club on your back. When you’re on the road, everyone knows you’re a member of the Hells Angels. In addition to that, it also depens on which dress code you have to comply with within a charter. For example, some do not allow their members to wear shorts or to wear color other than black.