Disqus is one of the leading comment systems. This add-on allows you to add a comment thread to every element, connection, loop, and map overview.
First, if you don't have a Disqus account yet, you can create a free account. After you are logged into Disqus, you can follow their Publisher Quick Start guide.
As you work through that guide, you'll set up a new Disqus site and be given a shortname for your site—you can find that below the box where you enter your site name, as shown in the image below:
Copy that shortname, then return to Kumu, and open up the Add-ons menu in project settings.
Activate the Disqus add-on by pasting in the shortname you copied before.
Voila! You'll now see the Disqus comment box on each element, connection and loop profile (as well as the map overview).