this kind of recording can be very tricky to get right, and i would encourage you to schedule some time with the group to do some experimenting with mic placements and trial recordings. listen to some commercial CDs of bluegrass music together and talk about what kind of sound they are after. as in any live-type recording of small ensembles, i recommend a very nice stereo pair out front (perhaps a pair of neumann km140s in ORTF for this), coupled with several accent mics for your individual instruments, all on separate tracks if you have enough. i might compress the string bass to give consistency to the foundation. track dry. a touch of medium hall reverb to the stereo pair at mix, combined with a little manual fader boost to solo lines from individual instrument accent mics without the verb to bring them more upfront during their solos. use the best mics you can get, and keep them pretty tight. the mics that work well for live performance are often not well suited to recording properly.