To add a new Site to Disqus within one of your existing Orgs, you'll want to register the site through the link in your Org level General Settings.
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First you'll need to go to your organizations general settings. If you are an owner for more than one org, you'll have to select which one you'll want to modify.
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Then, under Organization in the left rail, select General. After that, you can click Add in your basic organization information to create a new forum within that organization.