You’ve got the first step, announcing it and gathering folks. You need to pick a song and tempo. In the past we’d usually get things laid out for who was doing what on the rhythm tracks. Then, see how many folks want breaks and on what instruments. The cool thing about a fiddle tune is you could do about as many breaks as you want. You can also split breaks if you get too many folks. Once you have an idea of what you’ll do, someone can make a click track or jump straight to a rough rhythm track. You include a sharp sound at the front so whoever is mixing can line up the various tracks (this will make sense once you see the initial click or rhythm track). You collect the various tracks, mix it, master it and you are done.
We used Kompoz for a while. Kompoz changed and we skipped it on the last one we did. You can go either way on that (I haven’t tried Kompoz for a while… it might be improved). If you don’t use Kompoz, you can just use DropBox or something similar to exchange files.
Looks like you have a couple options for upright bass already. That’s awesome. Larry is in on it, that’s huge. He’s great at every part of it.
It’s helpful to have one person “driving the bus” to keep things moving. Larry would be a great choice because of his experience and talents.