You need to scan the QR code with your camera app. Most of the camera apps have inbuilt QR scanner. Once you scan the code, you will get the group name and the option to join . If your camera app not able to detect the code, just install a QR scanning app from playstore.

How would someone be able to join the group by scanning the printed qr code?
Do they need to install another app. Whatsapp only have the “WhatsApp web” feature to scan from pc/browser.

Thanks,
Shanny