And to add on yo what like_taking_diamonds said, you also talk amongst all the authors in a thread page in the forums under Group Written Stories. It's a bunch of fun. =D
A co-written story goes on both peoples story list and has both parties listed as authors, the people involved can all post chapters. All that has to be done is going to the authors button and you can add as many authors as you want if you know their username. :)
It's a bunch of fun.
=D