Writing a Group Novel
I did this very same thing last year with two dozen Prosers. The best way for a group chat is to create a post for all those who get involved, tag them all and start pitching your idea. When I did this it was a group challengve to finish a novel in 30 days which we did. It's called "Survival". 47 chapters with both Forward and Prologue, and 65,843 words. Each Proser has their name at the end of each chapter. You can expect a lot of debate, a lot of ideas but you have to maintain control or it can get out of hand. Keep everyone focused, keep it lively and interesting. It would bes best with whatever idea you have in mind to write the first chapter. Then designate other writers in order of each chapter and make sure with each chapter, all thiose involved are always tagged so they can who is next in line to write, what has been written so whoever is next has a strong idea of what to work off of. It's best to do at least one mass post each week so everyone knows what is going on and one thing you can do in the post, is highlight what each chapter entailed. In doing so, it can prompt more questions to getting the novel written. Continuity and clarity are key. If a character is 6'4 and 220 in one chapter and two chapters later he is suddenly 5'9" and 180, that presents a problem in uniformity. Good luck!
I tried to post this in the Beta version but it wasn't working. Or if it did work, I saw nothing to show that it did.
This is the link to the novel I mentioned: https://theprose.com/book/2939/survival