I don't do too much group riding to be honest, but I can compare from the times that I ride with a friend, vs. when I ride on my own. On my own, riding without anyone to draft behind, etc. I average just below 20 MPH over about a 30 MI ride (I've really been stretching to break 20 MPH avg. over that ride, but so far 19.6 MPH avg. is my best).
Riding with a partner to share drafts with increases my avg. speed to as much as 23-25 MPH avg., although it's hard to hold that out over the 30 MI ride (my partner tanks before I do, usually). It would be interesting to see how substantial a difference a pace line might make, vs. just riding with a partner. I would imagine the larger number of potentially fresher legs to share the time on the front would increase the overall avg. speed of the group.
The bottom line is, no matter what, cycling is still a team sport. If you want to top out and do the longest ride at the fastest pace, you're going to do it with a group, rather than on your own.
On the other hand . . . if you're really type A personality, like me, and group riding rubs you the wrong way, so to speak, then you might find the fall-off in speed a worthwhile trade for the serenity of the solo.
Cheers.