You can easily tag collaborators in a Google document so they receive an email notification.
To direct a comment at a specific contact, type + followed by the persons name or email address - a drop down list will appear enabling you to auto-complete from your Google contacts. You can also type @ followed by the persons name or email address to achieve the same outcome.
Adding someone to a comment will email them a notification containing the comment thread. It is important to note though that you should be aware of the sharing settings in the document you are working on. Adding someone to a comment will not add them to the sharing settings of the document. If you add someone to a comment who doesn’t have viewing or editing rights to the document, they won't receive a comment notification in their email inbox.