From your administration site, navigate to Customers > Manage Customers. This is where you will be able to see all of the customers that have registered for an account on your site:
Find the customer that you are wanting to assign a specific customer group to and click Edit to access the page where the website's user information and activity are listed.
On the left, you will need to click Account Information
From this menu, you will have the ability to assign the customer to a specific Group
In the example above, you will see that I have selected this customer to be assigned to the "Wholesale" group.
Click Save Customer to save the changes that we made to this customer.