Watch the Using a Capo to Change Keys online guitar lesson by Stephen Mougin from Bluegrass Rhythm Evolution
Capos are a required gadget for bluegrass, because we're looking for a full chord sound with open strings. Price can vary from inexpensive to pricey, depending on quality. The yoke-style capos tend to keep each string of the guitar at a more regular pitch, but make sure you put your capo on STRAIGHT and directly behind the fret.
Spend some time studying the most common capo positions (2nd fret G = A, 3rd fret G = Bb, 4th fret G = B, etc.) so you can join in a bluegrass jam. if you're in doubt, ask your bluegrass jam cohorts, they'll be happy to help.