If you want to, you can type a short note telling everyone why you're sharing or what you want them to do with the file. Click Send Invitation when you're done. When people accept your invite, they can see the document by clicking the link in the message.
To actually edit the document, your collaborators need Google accounts (or be willing to sign up for one; there's a link at the top of the page to do so). Viewers can read without signing up.
Tracking Revisions to Documents
Up to 10 people can edit the same document at the same time. With all these people invited to paw over your documents, spreadsheets, and presentations, you may want to keep track of who does what to the files. (Just to give credit where credit it due, as well as to have someone to blame for the lame changes.)
If you need to revert to an earlier (that is, less-ravaged) version of your file, open it and choose Tools->Revision History. A list of all the revisions to the file appears, showing the date and time of the revision, the changes to the file, and the name of the person who made them.
Click the version you want to see (choose Older or Newer to move backward or forward in the file's history until you find the one you want). When you find the right version of the file, click the "Revert to this one" link on the side of the page to make this version the live working file.
Tip: The Chat window on the Google Spreadsheets screen shows you who else is working on the file. You can also use it to instant-message your fellow collaborators about the changes everyone is making to the file. You may be on the same screen, but perhaps not everyone is on the same page.
Learn more about this topic from Netbooks: The Missing Manual.
Netbooks are the hot new thing in PCs -- small, inexpensive laptops designed for web browsing, email, and working with web-based programs. With this Missing Manual, you'll learn not only which netbook is right for you, but also how to set it up and use it for everything from job-related tasks like working with spreadsheets to hobbies like gaming and photo sharing.