Rory B opens the camera — not WhatsApp — and taps the link that pops up. Their own ladder page opens, already them.

iPhone: Share (on newer iPhones it's inside •••), then Add to Home Screen, then Add.
Android: the green Install button on their page, then Install.

This code is Rory B's own link — whoever scans it acts as them. Show it to them, not the group.