(Updated for Woffice > 2.5.2)
Here are the steps to allow Google Sign In for your Woffice website ;) If the registration are closed, only user having an existing email address matching the Google account's email will be able to be authenticated.
Firstly, make sure to enable it in the Appearance > Theme Settings > Login / Register > Google Options.
Secondly, you need to open your Google developer dashboard: https://console.developers.google.com/
If you do not have any project yet, create one and name it as you wish:
Click the "Enable API" button:
Select the "Google+ API" and enable it by clicking the "ENABLE" button.
You can now reach your "Credentials" page and "OAuth client ID":
The application type is "Web Application" and you need to set as "Authorized redirect URIs" the callback URL found in your Woffice Theme Settings (see first screenshot above).
Once done, you will find an unique Client ID and Client Secret that you can copy/past back in the Woffice Theme Settings.
Note that Google offers several option to customize the integration (logo and text).