Sharing pulls can make a huge difference. Generally speaking, I can average about 2-3 mph faster with the same amount of effort on group rides compared to solo. A lot depends on the group, however. It works best when the riders have comparable strength and endurance. If one or two riders are a lot weaker or in poorer shape, it can slow you down enough where a group isn't much faster.

My fastest long ride ever, about 22 mph over 60 miles, was a large group ride where I never had to take a pull. There were so many strong riders that I never got a turn at the front, not that I was avoiding it. I got spit out the back about the 40 mile point, but still managed to turn in my fastest ride thanks to a few other strong riders who got dropped about the same time as me.