With this extension you can sell / upgrade customer group same as regular product.
Customer need to be logged in. If he is not from customer group that you are selling at top he will see link Upgrade membership . When he will click on that link will be redirected to checkout. After completing the order and if he get to success page his customer group will be upgraded .
You will need to create product that will be 'upgrade membership' . Set this product id and customer group id in config file.
If you need help i can set this for you for free.
Demo http://demo.plusmotiv.com/store log in admin with admin / admin