Make possible create users and login via OAuth or OpenID
For OAuth you must create app for certain providers. Login button appear at login page if app id specified. Settings are in "Social login" section of settings page.
You can create app by followed urls. You can copy link of certain login button to get proper "redirect url" for OAuth app setting.
Details about "Allow login only from specified domain" google setting you can find here #44
Custom providers is on your own. Officially not supported
You can use 'social_login_auto_redirect' => true setting in config.php for auto redirect unauthorized users to social login if only one provider is configured.