I think Dana Jean has this right. Assuming I fully understand it, each person just continues on where the last person finished. That will be fun for me to read. What does the group do about the word count? Is there a limit per post? If there are a lot of writers, then maybe some sort of limit would be appropriate, but whatever people think is best. Like Dana says, probably best not to think too much about it. (As for the person who is the last author, it probably should be one of the professional writers in the group, but again, that's not necessarily to be seen as a requirement.)