There is a helpful video: Enabling Google SSO for your NodeBB forum - YouTube, which is good, although I needed additional steps, basically Google Admin.
Just in case it's useful for some people, using Google Apps accounts, after installing nodebb-plugin-sso-google, activating it, and reloading, I needed to go -with the admin account- to Google Admin:
https://admin.google.com > More controls > Apps > Additional Google services > Settings for specific organizational units > example.com > Page 2 > Google Developers Console > On
Then to Google APIs:
Click on Google APIs
View All > Social > Google+ API > Enable > Create
Project name: Example Login
Project ID: example-login
Create > Enable > Create credentials
Which API are you using?: Google+ API
Where will you be calling the API from?: Web server (e.g. node.js, Tomcat)
What data will you be accessing?. (x) User data
What credentials do I need?
Create an OAuth 2.0 client ID
Name: Web client 1
Authorized redirect URIs: https://example.com/community/auth/google/callback
Create client ID
Set up the OAuth 2.0 consent screen
Email address: email@example.com
Product name shown to users: Example Login
More customization options
Homepage URL: https://example.com
Product logo URL: https://example.com/content/images/logo-120x120.png
Continue > Download credentials > Download > Done
Web client 1
Client ID: (...)
Client secret: (...)
Creation date: (...)
And finally to the NodeBB plugin:
NodeBB Admin > Social Authentication > Google
Enter Client ID, Secret
[yes] Skip email verification for people who register using SSO?
Tested, it works well.
Starting v0.3.0, NodeBB will require a third party emailer to send emails on its behalf. This plugin allows NodeBB to use the PostageApp service to send emails.
npm install nodebb-plugin-emailer-postageapp
Be sure to activate the plugin in the admin panel and set up the proper API key.