I'm going to branch off the HTML5 GameMaker to make an HTML5 Group Project GameMaker.
I'm going to chop a lot of stuff out, when you make a project, you can add people to be able to edit the game. There'll be a chat box incorporated into the interface. You'll be able to see who's editing what at all times.
I can't guarantee a time frame, I'll probably get started on it sometime.