Watch How to Tag Contacts to a Comment in Google Docs:
Google users have the ability to notify their contacts of a comment within Google Docs. This is especially helpful when multiple people are contributing to a single document.
1. Within a document, highlight a specific area you would like to comment onSelect insert comment.
2. After writing a comment, use @ or + to bring up a list of contacts, and choose as many as you would like
3. Keep in mind that tagging a contact to a comment will not alter the Share settingsIf you would like someone to view or edit the document, be sure to assign the appropriate access in the Share settings.
Reference: Google Gooru - How to Tag Contacts to a Comment in Google Docs