I would like to put in a good word for the late Fred Hellerman.

When the Weavers performed their rousing arrangements
of traditional song (or otherwise),
Pete Seeger's banjo and occasional mandolin got all the accolades.
Fred Hellerman remained the true accompanist,
providing both rhythm and counterpoint,
complementing both Seeger's instrumental solos
and the vocals, both solo and choral, of the Weavers.
Those Weavers recordings were some of my earliest exposure
to sophisticated arrangements of popular/traditional songs,
and for me, Fred Hellerman is the unsung hero of those arrangements;
I learned much about accompaniment from listening to him.