Knowledgebase
How to disable and enable guest checkout in Magento
By default, in Magento guests can shop without creating a customer account on your site. If you want to, you can disable guest checkout and allow only registered users to purchase products from your store. You can do this from the admin panel of your Magento.
After you log in go to System menu>Configuration>Checkout button in the Sales section on the left>Checkout Options panel on the right. After you expand the Checkout Options panel you'll see a drop-down menu Allow Guest Checkout; just set it to No. Once you do that a drop-down menu will appear under it labeled Require Customer To Be Logged in To Checkout; set it to Yes and click on the Save Config button in the upper right corner.