Yo Punkin! First, you have to decide if you want to do it fast, or do it right. First, I would get a sense of how experienced they are with either studio mics or overdubbing. Traditional folkies often have the delusion that if they ignore all that gear, they'll somehow be more natural, so you often have to sneak up on them with a condenser mic while they're hugging a tree. (Sorry, I spent the whole weekend doing live sound reinforcement for the New England Folk Festival Association- It was a Martin, banjo, and fiddle convention!). First, listen to these people sing, just listen. You need to know what is the order of loudness, from strong to weak. And you need to know how good they are, in terms of timing and pitch, from best to worst. I'd start with a semicircle, like a stage, with SM58's or similar mics, 3 mics, three tracks. That's what they are used to, a stage with an SM58, in line, facing an audience. That will make them comfortable. Don't use reverb. If they are real folkies, instead of folk rockers, they won't be used to it. No EQ going in.
Next, move to plan B. Put an omni in the middle of the room and an X-Y coincedent pair of small diaphragms overhead, above the omni. Then move the strongest vocalist back a little, and the weakest one in a little. No headphones. Just let them sing. Later, you can compare the X-Y and omni tracks, and use one or the other, or some combination of the 2. Keep them a good distance from the mic, like 3-4', no pop filters.
Now it's time to point out that we have this new-fangled thing called sound-on-sound. Take whatever mic works best on the singer you have identified as the best overall singer. Give them a click track, cans, a pop filter, in the nice vocal booth. Still no reverb, no EQ. Then overdub the other parts with whatever mics work for the other singers. Then mix the crap out of it, EQ to taste, add *a hint, and no more* of ambience, and let them decide what they like. You now have essentially a live recording, a live studio recording, and a studio recording. Don't be surprised if they prefer the 3 SM58's. They are looking for a sound that sounds like a farmer singing in a barn, not the Kelly Clarkson vibe. But you never know. Depending on what they bring to the table, any of those versions might work.
Basically, the plan is to start with something that doesn't require them to be very flexible, or vary their technique much. With each step, you are introducing more variables that they have to adapt to. Many traditional folkies will be spooked by cans and a click track. The ones that aren't- well, they are called Nickel Creek. Good Luck-Richie
