Twilio Sync allows users to share their online status in an app using Express, Svelte, and Twilio Sync. To achieve this, you need to set up a backend server with Express, generate an Access Token, and create a frontend client that uses the Twilio Sync JavaScript SDK. The frontend will render a list of online users and update in real-time as users join or leave. The app also includes features like user sign-in and removal, allowing multiple users to collaborate simultaneously.