Didn't realise there was such an anti helmet
unfortunately there is & wouldn't matter how good an argument you put forward to show that they may help reduce serious head injury or worse as once people start making comments on thudguards & wearing helmets while in cars the chances of of making any reasonable argument for helmets actually being listened to is next to none.
at some point someone will tell you that helmet wearers are more likely to have accidents as opposed to none helmet wearers but I think given more people wear helmets that may explain why more helmet wearers are involved in accidents.
there will also be those who argue that wearing a helmet will increase the chances of being struck on the head simply because it makes the head area bigger, they will also not accept that you may still have been struck on the head regardless, only now you have more of a chance to walk away after the impact as opposed to being rolled away on a stretcher.
reasonable arguments just don't work.
bottom line, your friends want to join you riding your bikes, fine. You have every right to request that they follow a few simple rules, what if they don't want to? then let them ride by themselves.
After all they want to join YOUR group, your not asking to join there's & forcing them to wear helmets once you join.
your group, your rules!
I wish you, your friends & family have safe pleasant rides, may the wind be forever at your backs & keep the rubber sides down.
good luck