Google Docs is a free website by Google that allows users to create and edit presentations, spreadsheets, forms, and documents and share them and collaborate with other Google users. A Google user can create one of these and then opt to share them with other people. With the share option, the creator can attach email addresses to the document, and they can choose to edit or share the document with additional people.
We used Google Docs in class for a group project, and it was pretty easy to use. Two people in the group created documents--one created a presentation, while the other created a presentation. The rest of the group members told the two creators their email address, and the creators attached each of the group members to the presentation and document. When we all had access to the documents, we edited them and put our ideas into each. In this light, we all owned a piece of the document and all became "creators."
Google Docs is nice for group projects because it allows people to instantly collaborate and edit the documents created as opposed to emailing back and forth one documents each time edits are made.
If my description of Google Docs didn't make sense, try this video: Google Docs in Plain English