Firstly, from the File menu, select the item "Email collaborators".
NOTE: If you have not shared your document with anyone, you will be prompted at this stage to add collaborators.
This step also requires the document to be named (also required when sharing the document with others).
Once you select this option, you will be presented with a dialog screen that lists those with whom you have shared the file and a message window.
|"Send message" Dialog|
In this screen you can choose to email all document collaborators or select only some. Simply type in your message and hit send. You also have a checkbox available if you would like to receive a copy of the email for yourself.