The song of the week is 'The Crawdad Song' in the key of G.
The chord progression is:
(Prog. V2 on the basic progressions handout. In the key of G: 1=G, 4=C, 5=D.)
The melody of The Crawdad Song (see the attachments) has no built in pickup notes, so to start the song effectively with an intro break at a jam, one needs to create pickup notes for it.
When the song is played in the key of G, the first melody note is a G note. The simplest and most straightforward way to create a pickup measure for this scenario that consists of 3 quarter note pickup notes is to start with a D note and then ascend from there up the G major scale, playing an E note next (2nd fret on the D string), and then finally an F# note (4th fret on the D string) This should lead your ear to want to hear next the G note that starts the first measure proper of the song.
Other songs played at the beginner jam that, when played in the key of G, have a G note as the first melody note and have no built in pickup notes include Buffalo Gals, Down The Road, and Mama Don't Allow. This same set of pickup notes (D,E,F# when in the key of G) will work just as well to lead into a break for these songs as what it does for The Crawdad Song.
If playing in a different key, then the pickup notes will need to be transposed. For instance, for the key of A, each of the three notes need to be raised up by a whole step, for the distance from G up to A is a whole step. The three notes would then be E, F#, and G#, which will lead the ear to want to hear an A note next. For the key of D, the notes would be A. B. and C#, for these three notes have the same relation to D that the notes D, E, and F# have to G and that the notes E, F#, and G# have to A.
Here are some bluegrass versions of the song to take a listen to:
Flatt & Scruggs - key of E
Doc Watson - key of D
Randall Franks - key of G