I'd 2nd what
furiousferret said about volume and intensity. If your group rides are as intense as intervals would be, use them as your interval days. That'll have the additional benefit of being very specific, since you're training to do better on those group rides. If you find you want to ramp up the work, add intervals before or after.
I think it's pretty well accepted that more than 2 hard sessions a week doesn't add add'l benefit. There's also consensus that the days you designate as hard should be really hard. It's a data point of one, but over the last few years my hours have increased, and the increase has been in Z1 and Z2. I've got stronger during the same time period.