220-age is an estimate for average members of the population. But because it's only an average, it doesn't work for a number of people. It apparently doesn't work well for you.
The maximum heart rate that you see on rides isn't really a great way to set your training rates. I suggest getting a book like Carmichael's "the ultimate ride" which uses the average heart rate from a 3 mile time trial to set your training zones. You could also consider Friel's "The cyclist's training bible".
It's going to take you some time building up aerobic capacity. It's okay to push yourself in a group now and then, but if you do that too often you'll compromise your progress.