Well, I have never been a part of one of those, but..

my guess is that they'll advertise locally and within a couple hundred miles and word of mouth and social media factors in in greatly.. they'll advertise what song and probably what key (most musicians would do the same key as the CD, but some wouldn't have a clue), so everybody learns it and learns it in the proper key..

I have no idea how much they practice, if at all.. most times the songs are not overly technical songs.. I know this has been done with Smoke on the Water (Deep Purple) and Cocaine (Eric Clapton) which are both as easy as it gets to play - most guitarist state Smoke on the Water is the first song they ever learn to play.. What I always wondered was: most people are not gonna learn to play the same song the same exact way.. my guess is that these subtle differences get swallowed up by the sheer magnitude of the amount of instruments and just the basic melody probably shines through.. so, keep the melody simple and all should work out.. and I imagine overdubs are part of the game - overdubs being somebody goes in and adds parts later or cleans up the mix and what not..

I guess the hardest thing would be to have all drummers on the same beat - probably one is tasked with setting the tempo and everyone follows suit.. Like I said, never been to one, but have seen bands playing where multiple musicians would get up and play Cocaine, or something all at the same time.. same concept, just a ton more instruments.. I've actually never done that, either lol

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