Will it be strictly vocals a Capella, with an instrument, or background music? If it is just their vocals, you could boom it if you happen to have the extra hands to hold it, but a plain old mic stand would work too. If they are playing an instrument, I would mic them separately from the instrument, running each into their own channel. If they are using background music, I don't know what I'd recommend. You might want to play around with it, and see what gives you the best result.