Jim from Boston: I'm very leery of group riding with people I don't know, especially on large rides like this one (the highest bib number I saw was something like 4300) and especially when the roads are a bit wet. Perry (the triathlete guy I rode with) was much more willing, even though drafting is not allow in tri races - he doesn't do much group riding, either.
There were several wipeouts in the first 40 miles or so, when the roads were the most wet and when the clumping of riders was still pretty high.
Perry was very easy to ride with but he was so skinny and his Kestrel aero bike was so slippery that I told him he was pretty useless for me to draft behind. Conversely, he could ride behind me and hardly had to pedal, but while he'd save some watts, he'd be lowering his average speed!