Google have now announced the end of Google Friend Connect so I do not suggest anyone uses this method to add visitor comments to a Google Site as this tool will disappear soon.
Thanks to Jorek who brought this to my attention I have found a much more effective way of getting Visitor Comments on your site using Google's Web Elements' comment gadget, which means you don't need to register for Google Friend Connect.
Go to the Google Web Elements Conversation Element generator
Set the settings you want (Size, Topic and Scope)
From the code generated in the code snippet box copy the code give (an example follows below:
<!-- Google Conversation Element Code -->
<iframe frameborder="0" marginwidth="0" marginheight="0" border="0" style="border:0;margin:0;width:600px;height:700px;" src="http://www.google.com/friendconnect/discuss?scope=site&topic=Google" scrolling="no" allowtransparency="true"></iframe>
Go to the page on your Google Sites where you want comments and use the Edit page button
Use the HTML button and then paste the code from above, use the Update button (if you want to move or position the box do so) and then use the Save button.
You're done: please comment on the right.
Note: there is no moderation with this gadget unfortunately.
RSS Feed of my Google Sites How-Tos