It would really help if people give a little background about what they want to achieve asking the questions.
Some googling told me what disqus is and it also do brought me to this mod for SMF : https://www.idesignsmf.com/index.php?topic=244.0
I know your use case and I see why you may want it, but as far as I know this has not come up with TP before: we have our own comments for articles as you know...
By the way: It is possible to hack the comments system to allow guests to comment, though it is not standard.
Tbh your visitors do not want to register in your forum, why would the register on Disqus?