Delta Chat has a QR-code based mechanism for inviting people to a chat, without needing to first exchange phone numbers or e-mail addresses. You can use it to establish one-on-one chats or to let people join a group.
This HOWTO first explains how to scan QR codes on Android and Desktop, and then shows how to create QR codes for yourself or for a group chat. You can find it here: https://delta.chat/assets/howto/qrcodes_howto.pdf
We hope it’s helpful and would be happy to get your feedback. There is also - slightly hidden - a Feedback group in the Howto, can you find it? If yes, please join if you have questions or remarks! Have fun with scanning and chatting!