The Shantyboys were one of the good New York folk groups of the 50s. The best known of those groups was the New Lost City Ramblers, whom you didn't include in your original list. They were Mike Seeger, Jon Cohen and Tracy Schwartz, whose son now plays with Steve Riley and the Mamou Playboys.

The Tarriers, mentioned by Don Meixner, were Erik Darling (Weavers, Rooftop Singers), Alan Arkin (Catch-22), and Bob Carey. If I remember correctly, Alan Arkin also sang with a group called The BabySitters, which may have included Barbara Dane...memory is shaky here.

Why not go back to the Forties, and pick up the Almanac Singers (Woody Guthrie, Pete Seeger, Lee Hays, Millard Lampell)?

How about groups by singers who aren't generally known for fronting groups? Dave Van Ronk and the Hudson Dusters, Ian Tyson and Great Speckled Bird.