What about Office 2013 social collaboration features

Office 2013 is a social feature that allows you to invite co-workers or friends to edit documents together through a mailbox link. You can send an invitation link to a friend by simply filling in a friend's mailbox link and choosing the right to give it to a friend (editable or read-only).

The friend enters the mailbox clicks the invitation link, will open the SkyDrive in the browser, uses the Office Online mode to view the document, if you have given the friend to edit the permission, then he can use the on-line edit or opens word to edit together the document.

In response to this online common editing feature, comments on word 2013 will show that the operation is from that user.