There is a little known extra security feature in Google Docs that stops collaborators from sharing your docs with others or changing the permissions of the Doc
Here is how to make your Google Docs more secure:
When you click on the blue share button, there is a little link under the invite people by email section.
By default, when you share your Google Doc with someone and give them edit rights, they can add other people and change the sharing permissions of other collaborators.
To disable this option, click the change link (see below)
When you click this link, you get the following option:
Select the second option which will then only allow the owner to change the permissions.
NOTE: If you created the Google Doc, you are the owner by default.
What other security questions do you have? Ask them in the comment sections below.