Another writer and I are currently writing a screenplay together but we are doing all of the research, outline, and treatment in Google Docs. He lives in Los Angeles, and I live in Canada. 

It is easy if you have one person create a folder and then share the folder with the other writers. You do not need to pass documents back and forth. You simply log into docs.google.com, find the document, and start writing. You can leave comments on the document for the other writers to see, and you can even watch each other type if you are in the document at the same time. You can even draw pictures with a Drawing document and leave it in the same folder if you are trying to get someone to understand an idea or sketch a storyboard. I think it's a great collaboration tool.