I have posted many questions regarding this and don't seem to get it right. However there is a twist to this.
I think I succeeded in granting permission to the user 'quirverT', because when I tried to create a new account on my website, the error changed to this:
Server Error in '/' Application.
Login failed. The login is from an untrusted domain and cannot be used with Windows authentication.
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.
Exception Details: System.Data.SqlClient.SqlException: Login failed. The login is from an untrusted domain and cannot be used with Windows authentication.