What do you mean... create your own plot!?
Well on Shadows we have 3 SIGN UP sections (past, modern, and future). Within each of these sections are genres (Fantasy, Action Adventure, Science Fiction, Horror, Romance, Regular life, Animal, and Miscellaneous). You simply chose the category that fits the type of plot you want to make, and then create a template. Write an introduction that tells the basic plot, the setting, the types of roles you expect to be played during the tale, etc. Be as specific as you can, and then provide a form for the other writers to fill out in order to sign up.
For example:
Name: What's the character's name?
Age: How old is the charcter?
Species: What is your character?
Appearance: What does the character look like? (description, or picture, or both)
History: What is the character's history?
Personality: What is the character's personality?
Abilities: What are your character's abilities?
But I don't want to make my own plot...
No problem! Just go through the same process but instead of being the one to write the plot, be the one to fill out the form and sign up!
Where's the rest of the FAQ?
It's under construction! 
