Not to denigrate the fun of riding with others, nor the potential benefits, but it does seem that accidents increase when riding with others, as opposed to riding solo, for a lot of already well documented reasons, some of which I list here:
1) Trying to keep pace with faster riders
2) Not stopping when you feel the need so as to keep up
3) Being distracted or influenced by other riders for a number of other reasons
Despite the adage “safety in numbers”, I wonder if the statistics bear this out with bikes. It sure seems from my casual observations that many accidents are due to some type of interaction with another biker; whether by direct interaction such as the recent two bike Harrison incident, when it seemed they were running parallel around a two lane corner and perhaps at excessive speeds; or by indirect interaction such as not stopping to rest or eat when with others.