Google-drive – Creating/Editing Existing Documents

In addition to Chris' method, if all you require is access to a single Google doc, consider creating a new Document which is set to allow anyone with the link to edit (no-sign in required).

Then you just need to give the link to whoever you want to be using the document. It would be impossible to access the page without the link and you dont need them to login to edit the document.

This would also be beneficial because using this method the document remains owned by an employee rather than the guest account.

Almost three years later, Google Docs now natively supports the editing of DOCX files stored in Google Drive, though changes made to a Word document are not automatically saved. (That said, saving changes is a matter of clicking a button.)

If two people are working on the document at the same time, Docs will notify the other person if the document has been saved while they are working, but if the second person saves their changes later, the version saved last will overwrite the previous one.

Since Google Docs lets you know who is currently viewing the document besides you, if you notice another person working on the document, it would be of mutual benefit to communicate with your colleague—perhaps using the built-in chat system—to make sure that neither of you overwrites the other's hard work.