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:

  1. Right click in a document
  2. 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

  1. Follow the steps to comment
  2. Type "@" or "+" and begin to type someone's name. 
  3. 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.