To add email functionality to a Ruby app, you'll need to set up a transactional email service such as SendGrid or Mailgun, and integrate it with Courier, a visual editor that allows you to design and preview your email without having to write code. You'll also need to create an API key for your email service provider and authenticate your domain. Once you've set up your email service and integrated it with Courier, you can use the Courier Ruby Gem to send emails from your app. The process involves mapping your notification to an event ID, copying-pasting a code snippet generated by Courier, and setting environment variables to authenticate with the API. With this setup, you'll be able to start sending transactional email notifications from your Ruby app in minutes.