My brother and I, and our families, used to play at bluegrass festivals all over the upper and lower Midwest during the summer festival season several years ago. We'll still pick bluegrass at a monthly jam in the QCA every now and again.
Good advice from
@snarf , especially the capo. Learn to "walk" between chords, is another good thing. Impress them with a few blues licks in a bluegrass timing. (My brother and I usually have the traditionalists steppin' and fetchin' when we wind up doing something like Buddy Holly or Chuck Berry in a bluegrass style).
Take a look at guys like Ron Block, Norman Blake, Peter Rowan, Dan Timynski (sp?) and Del McCoury on youtube.
A lot like this community here, you will find bluegrassers to be very helpful, and not at all critical if you screw up.
I'll tell you a story, I've told it here before, but many years ago, 25 or more anyway, we were at a festival in southern Illinois. After the day's activities, my brother, his wife and father-in-law, some friends and my wife and I were sitting round our fire in our campsite just picking some tunes, telling off color jokes and laughing. When out of the darkness came this little old distinguished looking gentleman approaching our fire. He said that he loved to be where people were having fun and asked if he could join us. We said certainly, and he sat in with his mandolin. Well that old gentleman happened to be none other than Bill Monroe!