Kristy, my previous post asked for everyone to give examples of add-ons that they are using with shopify. You would have benfitted from that post (had people replied to it) lol.
The answer to your question is: Haloscan – commenting and trackback
www.haloscan.com (I think) It will allow users to post comments and you will have some administrative control over posts if you purchase the full version ($12) It was super easy to implement into the site. (Only two lines of code to cut and paste.)
Let me know if you need any help with it. It gets lonely around this forum.