Open: Discussion Forum Activity - dan7davis/Lambda GitHub Wiki
Option 1: Scrape the DOM somehow?
In order to see the amount of posts by a given user, you can go to this URL:
https://courses.edx.org/courses/course-v1:[course ID]/discussion/forum/users/[user ID]
and run this script:
Number($(".discussion-count")[1].textContent);
One solution could be to run a loop through every user ID's URL and run that line of JS.