Google has just announced a change to the way it handles shared documents:
http://googleblog.blogspot.com/2009/08/sharing-with-google-groups.html

This means that if any member wants to share a calendar, spreadsheet, Google
site or document with all the other members of the Travel Spain group, they
simply have to add the group's email address instead of all the members'
addresses one by one.

Simply put, you just need to add the email address "
travelspain@googlegroups.com" (without the quotes) to any of your shared
Google docs for all our members to be able to see that document and, for
example, participate in updating it.

Roger.