WhatsApp isn’t the only messenger lacking cryptographic assurances for new group members. In 2022, a team that included some of the same researchers that analyzed WhatsApp found that Matrix—an open source and proprietary platform for chat and collaboration clients and servers—also provided no cryptographic means for ensuring only authorized members join a group. The Telegram messenger, meanwhile, offers no end-to-end encryption for group messages, making the app among the weakest for ensuring the confidentiality of group messages.
Or Matrix (warning some assembly required)
That study was 3 years ago, features to create private (invite only) group chats are supported now.
Are parts of matrix closed source?
How can matrix be insecure when it keeps locking me out of my own messages?
I actually found xmpp to be a breeze compared to most Matrix solutions.
Synapse is bloated, dendrite sucks and conduit is in perpetual beta and the uwu forks die too fast.
XMPP: