The thread title intrigued me, so I took a look at this. I've done something similar, though not quite the same (everyone is taking turns writing the same story with the same characters vs. each person controlling a specific character and building a shared world together). I figured I'd give a few pieces of advice that might be helpful.

-Having a specific place to discuss ideas and plan things out can be really helpful. Discord is a good tool for this, for example.
-Establishing a little bit of an overarching plot or goal is helpful. If there is no direction, it can get confusing and people will go in different (plot) directions because they have different ideas of what will happen.
-Establish your genre ahead of time. E.g. if you're in a fantasy setting, your characters aren't going to ride into battle on huge mechs.

Just thought I'd share. I don't think free roleplay posts have enough depth for me to write in, but I will at least lurk here to see how this goes. Good luck!