Security isn't generally an issue because you can create custom URLs which the general public would have a very difficult finding. To do this, simply type in your custom URL at the end of the URL (see below where I've typed in gsd 2011).
As long as no one has used this custom URL in the past, you will be prompted to create a new pad. To share your pad with others, simply copy the URL and email it to them by selecting "Share this pad" and then "send an email invitation."
In the pad, there is a main window where the document is created. Each user gets a different color of type making it easy to distinguish who is doing the writing. In the bottom right corner there is a chat window where participants can chat about things not directly related to the document.
When you are finished you can export the pad in a variety of formats, such as a Word file, for later viewing. You don't need to export the pad, however, as it is stored in that URL forever (unless the site disappears).
Go ahead and give it a try here: http://www.primarypad.com/gsd2011
TOMORROW: VOICETHREAD! Don't miss the final day!