Microscope, by Lame Mage Productions, is an amazing game. It gives you all of the tools to create your own epic history, outlining the epochs and describing the events within. If there is one complaint, it is that the game only accommodates a max of 4 people per game session. When I was running it at Games on Demand at GenCon 2012, I wanted to share the experience with more people than the game allowed, so I started to hack.
I dragged in a total of 7 other players to play a massive game of Microscope. The eight of us came together with the broad concept and the palate of the game world. Once we had that determined, we created a total of three bookends. We created the Start Period, the Climax Period and the End Period.
At that point, we split into two even sizes groups. Group A explored the span of time between the Start and the Climax, while group B explored what happened between the Climax and the End. Both sides shared the Climax period and all of the Legacies. Most importantly, we had sporadic communication between the two groups. I asked the other group if they had established a particular important location, for example. We also notified each other about facts that we had established about the world.
For scale, we ran an eight-person game of Microscope in 2 hours. I limited each group to two new Periods between their respective book-ends, and we were able to flesh things out beautifully. The only thing that would have made it better was a full 4 hour time slot, so we could explore the setting in more depth.
Hope you find this helpful! I will have some updated information on Spark shortly.