If you would like new users to use your storefront without having to register, you may remove the Registration Form (highlighted below) from the login page.
To remove the registration form from the login page, please follow the instructions below:
1. Navigate to Admin > Configuration > Settings > Customer Settings. Click on the Registration tab and then the down arrow in the drop down box next to Registration method. Select the 'Disabled' entry from the drop-down list.
2. Save the configuration and log out to test the change. Your login page should now look like the following (assuming there have been no layout/skinning changes to your storefront):