Google Sites offers various ways to Sign In. You need to Sign in when you need to view (in case your Google Sites website is private or not public), edit or do any other operation in Google Sites. You can Sign In very easily by using any of the following way:
sites.google.com and use your log in details.
Just type "sites.google.com/a/domain-name.com" Change "domain-name" with your domain and use your Log in details to Sign In.