I've seen this work with a couple of different books, and I think The Night Circus would lend itself really well to people taking it in turns to write 2 sentences that will contribute to a story like it.
The sun was down, the lights of the circus just starting to light up. It was new to town; everyone was really excited for this first night.
From the outside it appeared to be just another average circus, but once you walked inside, the red and white striped tent transformed into a world of magic. It was a brand new experience beyond anyone's wildest dreams.
You could already feel the magic seeping in your bones and reviving you. Maybe not the magic that involves wands and spells but the kind that makes you feel every nerve coming back to life. You buy the ticket and walk past the gate.
All except one, one particular piece was grotesque and ravaged, a talking point that nobody could seem to leave behind. Every where the circus went the item appeared like an unwelcome guest, it became something to be feared, something that scared children at night and unnerved adults.
How old you ask? How long is a piece of string? There was nothing at all to determine it's age, the design did not fit in to any era. Was it built? If so, by who? No one ever claimed it, no one could identify it. They figured the answer would never be found out....until one day there came a visitor...
The sinews of that particular piece which looked like cogs lined with slime. "Magnificent piece, isn't it?" The old man turned right to answer the voice that adressed him only to find...
The dwarf didn't seem entirely there and through the old man's foggy glasses, he could make out a small face with delicate features, bronze skin and small sly smile. "Would you like to meet the person who built it?"
I looked around unsure. My eyes told a different story he held out his and as if the old man knew I wanted to unravel the mystery to this mysterious place. "Yes I would."
The sun was down, the lights of the circus just starting to light up. It was new to town; everyone was really excited for this first night.