Condolences to the Family and Friends of all of those who were affected.   I used to ride in the past and can attest to the challenges being on the road on a group ride.  Without observing the how the ride was conducted those on bikes need to be applying MSF precautions for a safe group ride; staggered formation, proper use of hand signals, proper following distance, etc.)  Consider the use of modulating front and brake lights.  The abiity and competency of a rider needs to also be considered...it just takes one that can mess things up.  Lastly this does not prevent a careless driver of an auto to endanger a bike rider (s), however it may give the added margin of safety.  Ride Safe!