Einkauf is a distributed Shopping List.
Host this website, install it on your phone via "Add to Home Screen", pick a fancy key name, send the key to others. Boom done!
Through the magic of WebRTC and distributed service discovery, you don't need to worry about finding each other. As long as both people are online, changes will be broadcast directly from your phone to theirs, good for last minute 'Ice Cream' requests.
Works offline, syncs when coming back online.
Einkauf is a play on the german word for shopping list: "Einkaufszettel", you probably understand why the name was shortened, :P.
Using the pwa-minimal template and includes a small nodejs based server, so you can simply clone the repo, start the server, install the PWA on your device and shut it back down. No need for monthly costs of hosting. (Also look for it soon on my github page dletta.github.io)