I love add-on writing projects, like our own Two Word story.
There were two online Exquisite Corpse sites that I loved, Fuzzy's and Delicious Demon, which were both over-run with spammers and died.
Today I found a relatively new one, called Smashed Clock (http://www.smashedclock.com/exquisite_corpse/). It looks really nice, so I hope it lasts.
To add to an existing story, you choose the Existing Corpse icon, choose the title of the story you wish to add to, and write all you care to in the box. You can then put in a nom-de-plume and opt to have the completed story sent to you by email (I prefer to come and read them online, so I opt out of that.) Then you can choose "don't publish" to leave the story open for others to continue, or "publish" to end the story.
I hope our creative writers here will enjoy this site.