By default, all editors of your shared Google documents and document collections can share those items with others. In most cases, this is convenient, but if you have a document or collection that you want to keep confidential, you may want to change this setting to prevent accidental sharing.
To change the setting, go to the sharing settings for the item, and look for the line at the bottom which says “Editors will be allowed to add people and change the permissions” (marked with a red arrow in the image below). Click the “change” link and you’ll be able to prevent editors from sharing.
Of course, like many other methods of protecting documents from being copied or shared, this isn’t foolproof. Someone could copy the text of the document, download the document, or even just take a screen capture (anything you can see on a computer screen can be copied this way). So, along with changing a setting like this, it’s best to set expectations and let your fellow editors know who should see a document.