I'm currently getting ready to run an Open Party/West Marches style game with a pool of about 16 players. We'll be using 5e, but my question is system-agnostic.
I of course started with Ben Robbins' "Grand Experiments: West Marches" blog posts (here: http://arsludi.lamemage.com/index.php/78/grand-experiments-west-marches/ ), but after reading those and Justin Alexander's "Open Table Manifesto" https://thealexandrian.net/wordpress/38643/roleplaying-games/open-table-manifesto I'm having a tough time finding anything else experiential. Pretty much every other resource I find references one of those two to define the playstyle, says they think it's a neat idea they'd like to try, and stops there.
I can't say I'm shocked; building a sandbox like this is a lot of work and I don't know that I'd want to go to the trouble of annotating it for strangers on the internet. It's almost shocking that it's happened twice.
That being said, I'm specifically looking for things similar to Ben's West Marches: Running Your Own http://arsludi.lamemage.com/index.php/94/west-marches-running-your-own/ where a GM who has actually run an Open Party/West March style game for some time gives experience-based advice.
Of highest import is anything that gives insight on managing in-game vs. real-world time with multiple parties.