When Workingman's Dead was released, it was a 180 degree turn into Bluegrass. how could they make such a jarring change like that so quickly?
Jerry Garcia started off as a banjo player, and one part of the roots of the Dead were in a jug band called Mother McCree's Uptown Jug Champions. So I would guess that Garcia knew his bluegrass pretty well even before the rock thing came along.
It was through hanging out with David Crosby and CS&N. Workingman's Dead isn't bluegrass, it's 'wooden music' based on songs with three and four part harmonies.