For me the answer would be to do interval workouts solo, and then treat the B ride as a social/recovery ride. As others have said, you can handicap your equipment with higher rolling resistance tires, lower gearing, etc. The kid who won the local CX series would ride the local group ride over the summer on his cross bike with gravel tires. He said he had a road bike but liked his cross bike better, even if he was giving up watts.