I was involved with starship adventures and it started out as a short writing jam (that kept going). We basically got together and decided on the theme, stats, a selection of main characters and what they’d be like. We then each wrote an “episode” or two, (much like a self contained TV episode) and linked the episodes together into an overarching plot. (Major props to @Felicity_Banks in particular for co-ordinating it, it was a bit of work.)
Although it was a “whole story”, you can read it as individual episodes as well and we do have an episode list you can jump to at the end of each. In terms of other characters that appeared “transiently” in each episode, you could introduce whoever you wanted as long as they fit the theme and some of them got built into the main plot in one way or another and appeared again. We had a discussion thread going as we were writing and editing as well. I liked it 
If you mean MC character creation, we kind of set the MC up to be your retro TV starship captain, so there was some pre-characterisation I guess you’d call it, to make sure it fitted into the senario.
You’d need to ask someone who wrote on the bookshop story for more info on their editing/writing process. You’re given an MC with a background and a goal to go forward from memory.
Unless you’re going to have self contained stories, I’d say it’d be helpful to have at least some guidelines/background as to who the main charaters, including the MC are so that everyone is on the same page and you don’t have to make major edits to get the stories to fit together.
Edit: If this is of any help, this was the original thread started for starship adventures where some of the initial stuff got hammered out. (More happened later but this could give you an idea of how we started setting it up ) 3-day game jam in a shared 50s scifi world