Commenting in a file is a good way of bringing a collaborator in a document up to speed in a focused way.
To add comments:
- Right click in a document
- Select Comment
Tagging in comments:
Tag a collaborator to get feedback. People can reply to comments to create dialogue while looking at a document
To tag a someone in a doucment
- Follow the steps to comment
- Type "@" or "+" and begin to type someone's name.
- Optional: You can assign comments if there's specific tasks you would like someone to complete
For more information refer to this Google Support article.