Bring what you can and hope someone brings what you need. With luck there will be a stash pile on hand, but start scrounging now and be prepared. Have small parts, chains, lubricants, nuts and bolts, etc. if you want to be safe. Think like we're trying to equip an army of cyclists of every size and shape to cross the salt flats, and that there are hordes of mutant trogs along the way that will kill a quarter of the people and fuck up an unknown number of bikes, so maybe we'll have more bikes than people, maybe less. So, we want variety and redundancy.
Steel is real.
Coaster brakes and one-speed hubs are great, we don't need shifters or cables. Old 3-speeds that have stood the test of time, of course. In fact, everything of a variety of sizes, as much as possible, that's helpful when you're trying to, for example, change how far forward someone's butt is from the rear hub. Finally, piles of stuff will save your life when you need to canniballize nuts and bolts and stuff, or else you end up with a bike that doesn't run because you can't find the right thread axle nut. Extra frames can be used for tubing.