When you are logged into Google Docs, and you have chosen to share a document with someone else you have the option to work collaboratively on the document together. This means that you will be able to make changes to the document, but Google Docs also allows you to chat with other users who you have shared the document with.
For example, I have shared a document with a coworker. They are editing the document at the same time I am. When this occurs you will see the words 1 other viewer right below the comment box. When you click on the viewer text a dialog box will appear where you will be able to chat with the other online user.