For document collaboration I use Windows Meeting Space, although it obviously doesn't allow it to be read/edited online, just any staff member at the meeting sends the file to every person attending with the click of a button, and from thereon, everyone can edit the file live.
Obviously WMS doesn't use an online auth system either, you just open the "invitation file" for each meeting and you get added to the meeting automatically, so it still is private, but a hassle to get all the invitations out if you're hosting the meeting.
Oh and just to rub it in... great topic name!