In order for the plugin to work you need to visit the Google API Console and generate an API key for your site. You then need to add this information to the plugin settings page.
Settings up shouldn’t take you more than a few minutes. You just need to copy your details over. You can also set the page the user redirects to after logging in.
The ‘Login With Google+’ button is automatically added to your registration/login page. The button can also be added to any page on your website using shortcode or PHP code.
WP Google Plus Connect should encourage more people to sign up to your website as it makes saves visitors from entering their details. Existing users can also login via Google+ and then link it to their existing account.
I recommend checking it out if you are looking for a way to make streamline your registration process for users.
Link: WP Google Plus Connect