I've had decent results doing bluegrass bandswith a AT-4033 but it's cardiod, meaning it only picks up from the front of the mic. I just had everyone stand in a semi circle in front of the mic.
I tend to use a Studio Projects C-1 for just about all my acoustic instruments except the autoharp. I think an Oktava 012 does a little better job with the mandolin, but not enough to make it worth changing mics when I'm recording.
Whatever mic you choose, sounds like you need one with an Omni pickup pattern if, in fact, you and the other musicians will be standing around the mic in a circle.