Great song. I just listened to it for the first time on Rhapsody. This is a straight-ahead rockin' bluegrass song (even if they do have a drummer )and it uses only the three major chords in the Key of A. The chords are A, D and E. The A major chord which suits this song best on the mandolin is the 2-2-0-0 "modal" A. The reason you might hear an Em in there is because they are employing the blues scale for their vocals, so the 3rds and 7th are flatted, but A,D & E are the only chords in the song. You can use the 7ths of some of them as you see fit, but they aren't necessary.
"I thought I knew a lot about music. Then you start digging and the deeper you go, the more there is."~John Mellencamp
"Theory only seems like rocket science when you don't know it. Once you understand it, it's more like plumbing!"~John McGann
"IT'S T-R-E-M-O-L-O, dangit!!"~Me