Riding with group provides the dynamic I can't generate on my own.
On an avereage day, I climb faster and cruise at a higher speed than riding on my own.
exactly. Most riders riding on their own won't push themselves beyond a "comfortable" pace. Riding with a group of better riders will drastically improve your ability.
It can be tough when you're riding with a better group and they are all talking while climbing and you can just barely breath, never mind talk!