Cataboo has some good suggestions. I've ridden with people where I left them on the hills (or was left myself) and then regrouped. Granted, I was riding with people whose egos were not easily wounded, so that helped.

That said, it is just one ride. There will be more centuries in your future from the sound of it. I would resist approaching your riding this year with this one ride as the end all be all. Sign up for another century this summer to do on your own or with your other "faster" friend.