Here are the 3 common major chord shapes used for vamping on the banjo - Root Shape, 1st Inversion, and 2nd Inversion. The banjo is tuned to one of them - the second inversion or bar shape. The Root shape, also known as the F shape, is another. Lastly there is the 1st inversion shape, also known as the D shape.
The pattern can clearly be seen when we play through the root shapes and inversions for both G and A
So much of Scruggs style banjo playing is based out of chords. It's very important to memorize these chord shapes and patterns.