At this moment 'Sign-in with Google' is limited to log in existing users of your community. Registering new users with a Google account is on our roadmap.
Before you can enable 'Sign-in with Google' for your community, you must create a client ID, which we need to call the Google sign-in API.
To create a Google API console project and client ID, you need to go to https://developers.google.com/identity/sign-in/web/sign-in and click the button Configure a Project.
You might need to login, then select an existing project or create a new one. You can name it anything you want.
Next, you enter the name the user will see when they are shown the Google login consent screen.
The last step is to obtain your client ID is to enter the URL of your community.
Now copy the client ID and paste it under 'Users' in your 'Community settings'
Finally, click save and you're done! Your users can now choose to login with their Google account from your community login screen.
There is more you can add to the Google login consent screen from the Google API console, like your logo or a link to your privacy statement.