I don't remember how it came up a couple weeks ago but a friend happened to mention the song or the movie, and I decided I had to go looking for a copy of the DVD. I remember seeing the movie once in the early seventies, but haven't since.
Based on a true story, Arlo Guthrie plays himself, Pete Seeger plays himself, Officer Obie plays himself, Alice Brock (the real Alice) plays Suzy, a minor role. It was actually filmed in Stockbridge, in the real locations described in the song. An actor plays the bedridden Woody Guthrie, who died of Huntington's Chorea in 1967 (the movie's events take place in 1965).
It reminded me of The Return of the Secaucus Seven, with a bunch of old friends coming together for Thanksgiving in an old church one of the couples has bought.
It was amateurish, and partly poorly acted/scripted, but a real period piece. Made me nostalgic for the late 1960s when everyone thought we could change the world.
There is a GREAT long review of the movie and its cultural significance, the first review listed, on IMDb.