The popular messaging app, WhatsApp has added the ability to invite someone to join a group chat with a link. Any WhatsApp administrator who has joined the beta program (go to Google Play and become a beta tester for this app) have this option in the latest release (or download the APK version 2.16.281) which lets you share the link directly using a QR code or NFC tag in this new beta changes. The admin can later revoke a link ultimately denying ability for anyone to join the chat.
To try it out, go to add a new participant to your chat goup, you will have a new "invite to group via link" option at the top of your contacts list. Tap on it to open a new screen with the unique link to your chat, which you can share via WhatsApp or copy & share using another app, or revoke.