Until that one comes out, here is a rather short documentary (2 minutes), explaining, among other things, why lead singer Billy was kicked out of the group by his father, and what it was like after the group dissolved in 1971:
This one is even better (includes a few seconds of them performing "The Rain, The Park, and Other Things" live). It's a little over 4 minutes:
It all started with the pure, bubblegum-pop psychedelia of "The Rain, The Park, and Other Things." This is a 1967 promo film for the single (it reached #2 in the U.S.). The footage is rather bad, but I love the song so much, I can bear it:
Here they are after they reunited (minus Barry and Billy) in 1990, appearing on The Joan Rivers Show. This is a great interview, and it is the one in which Susan voiced her frustration as to never being able to figure out which of the two girls on The Partridge Family was supposed to represent her(!).
Returning to their "glory days," here they are performing on their November 23, 1968, TV special A Family Thing. The song is "Indian Lake," and as pointed out at Bedazzled!, Susan is in fact quite a groovy dancer. I figure she must have watched Laugh-In every week.
There's a great summary of the special (including transcripts of each skit) at this Cowsills site.