Google this week opened up collaborative tools for mobile users of its Drive app.
Whether you use the Google Drive app or use Google Apps as part of Google for Work, you can now comment on docs from the Android and iOS apps for Docs, Sheets, and Slides.
You can also add collaborators to a conversation by typing their names in the comment box. The person will be added to the conversation and receive an email. Using a + or @ sign before the person's name also brings up their email address.