Between Meetings is a feature found on the Premium and Ultimate plans.
You can add a comment to a Between meeting report in two ways.
When someone else adds a new comment, all Board Members will receive an email notification about it. You can reply to that email notification with your comment (an example from Outlook is below).
2. You can also open the report directly to leave a comment. If you click on the blue report title in your notification email, it will open in BoardPro, where you can also leave a comment at the bottom.