There's more to riding than aerobic fitness, leg strength, FTP, etc. So, if you say your goal is to get better at group road rides, the best way to get better is do more group road rides and maximizing your speed on real roads and riding in groups.
I've found that indoor training at Zwift is better training for increasing my FTP than riding on the roads around here. That translates into me being able to do longer outdoor rides more comfortably, but doesn't translate into much higher average speeds over long rides - traffic conditions and getting through traffic lights/stop signs have a big effect.