Using the built-in Comments feature within Google Documents is another great way to collaborate. By sharing a Document and giving other people "Can Comment" privileges, they will be able to insert Comments into the Document, but they will NOT be able to change any of the actual content of the Document.
This is a terrific way for students to peer review each others' work without any worries about work being lost or modified. It's also a great way to get feedback on a Document without losing control of the actual content.