Helmets required? Who sez?
I've never seen a rider show up for a race or charity ride and try to insist on riding without a helmet. It just isn't done. Unlike races or charity rides, many group rides have no governing entity and may just be known as the "Tuesday 6 O'clock". Folks just show up and you can choose to ride on not depending on the mix of riders. Club and shop rides land somewhere in between in that waivers are seldom required, but all include the caveat that riders wear a helmet. There is some implied liability when any formally recognized entity organizes an event under the auspices of their business or club, so they have the right to set some conditions for participation. Other exclusionary conditions may include the wearing of headphones and the use of aerobars, both of which can compromise safety in a group. While most folks easily agree to any of these conditions, if for no other reason than that they usually ride that way, but what happens when somebody drops in and doesn't follow the rules-of-the-ride? If you're a ride leader, official or self-appointed, usually if you will remind the rider of the rules of the ride, the rider nods and says, "OK", and rides on, often to show up the next week with his helmet. If a rider just rolls out with the pack on his bars or with his earbuds blaring or without a helmet, about all that can be done short of a stick in the spokes is simply to express disapproval and ride on unless their behavior threatens the group. Most folks really don't want to be where they're not welcome and your cycling friends wouldn't put you in the position of having to say something to them.