At the time, joining groups in Signal was an involved process and the friction was higher than "a volunteer lead stands at the check-in desk and shows the QR code to each volunteer as they arrive for the day." Plus, most people didn't have Signal installed; almost everyone, and realistically the "almost" doesn't apply any more, has WhatsApp.

Signal's new groups work much more akin to WhatsApp and maybe I can bring it back up again once the pandemic has fully subsided.