There is a current bug with Google Docs & Spreadsheets, with many users unable to share documents. On Minstrel Hall, this is a bit of a problem for us, as character sheets have to be shared between a player and a DM. But fortunately there seems to be a workaround.
Shorten the document name, then share it. Once both people have access, rename the document to a more appropriate name, the sharing will be retained.
Courtesy of a user on http://www.google.com/support/forum/p/Google%20Docs/thread?tid=100dda558d2f9df5&hl=en&start=40