In various places of the forum, there are blue envelope icons standing next to the names of people involved in the review process (Authors, reviewers, Editor). The ones that you will be able to see will depend on your role in that article's review. In the screenshot above, the one for the Associate Editor can be seen.
Clicking on this envelope will open a 'Compose message' interface. It will preselect the recipient depending on the envelope clicked but this can be edited. However, only people involved with the review can be contacted and it is not possible for the authors to contact the reviewers or vice versa.