On the collaboration site, you post screenshots of the newest versions and graphical assets for the next iterations of the product. Also, you include the documentation, use cases, and problem descriptions.
The site, when well organized is an excellent tool to update customers on work in progress: "Can you look at page 3.a - product details?"
We try to create a page for each piece of functionality.
It is important to put the most recent screenshots on top of the page, and it is easy to see previous versions as you scroll down. Other, non-visual files can be attached.
Another trick is to set page layout in two columns, which Google sites allow you to do easily. The left column is for developers to post the current state and the right column is for artists to show the vision of what it should be.
A great site saves a ton of confusion and delays. We literally prohibit our people from sending Word Docs, Images, Spreadsheets, etc. via email, or Skype. All useful documentation should be posted for sharing with update alerts and revision history available.
