I have been editing my students NaNoWriMo Stories in Google Docs. The easiest way to communicate with them is to write comments. Since we are in the ‘it must be perfectly polished’ mode, I’m making a lot of comments! I’ve invited other teachers to give their input as well.
When I find an area that I would like to comment on I highlight the word or area with my cursor.
I can then either navigate to the toolbar at the top, find ‘Insert’, then scroll down to comment. OR I can locate the ‘Comment’ button on the right side of the toolbar, select it, then choose the comment button from there.
There is also a keyboard shortcut – Option + Command + M (Mac) or Alt + Control + M (PC and Chromebooks). Any of the 3 options will then bring up a comment window on the right side of the document. When this appears, type your comment, then select the blue ‘comment’ button. When the reader clicks on either the comment or the highlighted area, the highlight becomes a bit darker and the comment window becomes more prominent. 
Happy Commenting!
Nowa Techie 