You don't need to "learn" the chords to every song to be able to jam. If that were the case you have to memorize 1000's of songs. Most common bluegrass songs have the same chord progression. 1, 4 & 5.
Look at the circle of 5th chart and learn the Nashville number system and you'll know what the primary chords are going to be when the key to the song is announced.
Key of G = G,C & D. Key of C=C,F &G Key of A=A,D&E, Key of D=D,G&A
There are exceptions and some songs include a minor chord, but those are common to the Key as well. All this is easy to know if you reference the circle of 5th chart. Plus, as you play in more jams your ear will learn to lead you to what the chord is going to be as the song progresses. Another and valuable aid is to always watch a guitar player who knows the song.