I wrote a post for MapMyRun recently on ways to include group riding in an effective training routine.
The trouble with group rides is that they are often not specific to the goals we set, they are often moderately hard but missing the specific work that many athletes need to do to improve. I work with masters aged athletes, most of whom have goals in the off-road disciplines (mountain bike, Cross, gravel). Most of these age-group cyclists are understandably drawn to the social and/or competitive nature of group rides; it is how they are spending their limited personal time. The trouble for these specific off-road athletes is that they often misspend their energy on-road group rides, then wonder why they are too fatigued to do an appropriate volume of mountain bike riding.