Group riding is fun...being part of a giant fast flowing amoeba that changes shape, etc... fun to be part of a greater whole. It's fun to tuck in behind another and enjoy the draft while staring at his butt. Fun to take a turn at the front. HOWEVER.... we all agree you have to (1) know and trust other riders if you plan on being right on their wheel, and (2) be somewhere in the fitness range of the group. We all know the axiom about riding with others above your level to increase your fitness.....but, it's no fun getting spit out the back wheezing after half a mile. Or, dragging along with much slower others unless it's a "social" ride.
I learned about carefulness when the guy whose wheel I should have been watching more carefully jinked suddenly causing a wheel touch which sent me to the emergency room with a broken pelvis. Lesson learned.