Share 'How do you learn the Intro to a song?'

I joined a group that plays and sings folk songs (Kingston Trio, Bob Dylan, etc.).  We sit in a circle and each member selects a song from a substantial song book the group put together over time.  There are a few folks that play all the "Intros."  I'd like to learn how to play a few.  How do most people learn to play an "Intro" to a song?  Is it trial and error?  Are they written somewhere?  I'v…