A group ride is kind of like being 6' (or whatever your spacing is) from a rider where both are still, but are probably breathing heavily, and you are downwind in a 20-some mph wind. There are a lot of warning signals in that scenario. They are partially mitigated by being outdoors, if it is a big group your exposure is 'spread out' across a bunch of folks rather than one possibly infected person, and there is probably a lot of dispersion going on.
I doubt that anyone has done the analysis to properly assess this risk with a decent degree of confidence. The stuff that I have seen was not convincing to me. So my choice given those unknowns is to pass on that scenario and I am riding solo. It is helpful that this is my norm anyway. But it is going to force me to pass up a couple of century rides this summer that I was looking forward to.
dave