The Matrix server is open source. https://github.com/matrix-org/synapse The Element client is also open source. https://github.com/element-hq/element-web

It does not actually need to be configured in a federated state and frankly scales better when it's not. The login can be tied to anything or use it's own. From a modern SAML SSO to an old school forum.

You can run one for a few friends and it scales just as well as a private discord for a few friends. Just need persistent storage for media uploads if people are sharing video a lot.