Google recently added a feature to Google Docs, Sheets, and slides that allows you to tag other users in a comment. When you tag another user, they will get an email notification that they were mentioned along with a link to the document. If the tagged user is not currently shared on the document, Google will prompt you to give them access so that they can view your comment. This is great when you would like to have a colleague chime in on a conversation you are having in the comments.