Unfortunately (and fortunately privacy wise) that isn't possible.
Whatsapp Web doesn’t actually give your browser full access to every participant’s phone number anymore. Since WhatsApp switched to its multi-device system, many group members are identified by random “linked device IDs” (those ending in @lid) instead of real numbers. These IDs are designed for privacy, and the actual phone numbers simply aren’t sent to the web client unless you already have that person in your contacts or you’ve chatted with them before.
When you open a group’s info panel and look at the participant list, WhatsApp briefly fetches and decrypts that data so you can see the numbers in the interface. But that information only exists in memory while the panel is open — it’s not saved anywhere like IndexedDB.
So technically, you can scrape phone numbers from the DOM after a user opens a group and scrolls through the full list, but you can’t do this automatically for every group. WhatsApp’s encryption model is designed to prevent that kind of bulk access. In short, you can export what’s visible on screen, but there’s no reliable way to programmatically fetch all participants’ phone numbers across all groups. That is unless you start simulating user interaction for each group.