The author wants to build a group messaging app using Twilio SMS and Ruby, allowing him to broadcast messages to friends and family while maintaining an easy-to-maintain phonebook in the form of a Google Spreadsheet. The app will retrieve contact names and numbers from the spreadsheet, forward incoming messages from the phone to all contacts, and forward incoming messages from contacts to the phone. The author uses Sinatra as the web framework, Twilio Ruby gem for Twilio integration, and Google Drive's JSON backend to store and parse data from the spreadsheet. He also defines methods to sanitize phone numbers, handle Twilio's POST request, generate instructions on where to forward messages, and send messages to contacts or himself. The app can be deployed to Heroku, and the author provides step-by-step instructions on how to set it up and use it.