Mattermost is an open-source, self-hostable online chat service with file sharing, search, and third party application integrations. It is designed as an internal chat for organisations and companies, and mostly markets itself as an open-source alternative to Slack and Microsoft Teams. (Wikipedia)
There is an Mattermost server running at mm.let.rug.nl.
This server is primarily for members of Computational Linguistics in Groningen.
To get access, contact Federico Pianzola or Peter Kleiweg.