For a few months now, GEMS has been sending final decision notifications to reviewers. As of this month, GEMS is now including coauthors on final decision emails for all manuscripts. I just got one (as coauthor of a now-accepted paper in JGR Space Physics), so I know that the new feature is working. Both of these are small changes but ones that hopefully will allow the reviewers and coauthors to be aware of the final decision on a manuscript with which they have been involved.
For the reviewers, you are providing expert advice for the editor to take into account in the decision about the manuscript. Your recommendation for that decision may or may not be heeded. And, while your review comments are always forwarded, your decision recommendation is usually conveyed to the author, as that line in the review can be deleted by the editor if desired. Plus, we usually get two or even three reviews for each paper. While we greatly value your input, it is our responsibility to make the judgment on the fate of each of our assigned manuscripts. So, my comment to the reviewers: if you have any questions about the final decision on a manuscript for which you served as a reviewer, then please reply to that notification email (i.e., to email@example.com) and ask us. Without revealing names of other reviewers, we will fill you in on the rest of the story about the manuscript.
For the coauthors, you should be in contact with the corresponding author and should normally be aware of the current status of all papers on which you are listed as an author. However, we understand that sometimes the corresponding author forgets to forward the final decision letter to the coauthors, leaving them in uncertainty about the status of the manuscript until they perhaps see it in the table of contents alert. Except under unusual circumstances, coauthors should not contact the Editor. Rather this should be done through the corresponding author. So, my comment to coauthors: if you have any questions about the final decision on a manuscript, then please contact the corresponding author and direct all questions back to the editor or the journal through that person.
I hope that you find the additional communication from AGU to be useful.