Here's a suggestion:
Use the forum to create the list of people. Rather than everyone posting their update in the forum, why don't you trade the GMZ via messages and then cross them off the list of people in the post and include their version of the GMZ (And an exe of the "game") as a download link?
This will mean that anyone who's interested in contributing will have to get added to the list, they will only ever be working on the 'latest' version of the GMZ - and all of the GMZ files will be available as new edits come out.
This will stop version discrepancies, as if User1 starts editing File1, and User2 starts editing File1 at the same time, one of them will have an out of date version at the time of posting, in essence.
EDIT: Sign me up, though. This sounds like it could have some hilarious results.
EDIT EDIT: I feel as though a few rules would need to be added. I.E. No drag-and-drop, and comment your code.