What is Matrix?
Matrix is a decentralized, federated chat protocol. It supports all modern features that you expect from the likes of Slack, Discord and Telegram and also supports bridges to various other protocols, such as IRC, Slack, Discord, Telegram and Gitter, to name just a few.
NetBSD on Matrix
How do I join?
Using a public server
Sign up to any Matrix server and join any or all of the rooms and optionally the Space.
While matrix.org is a popular server, it is too popular and risks centralization, which also often results in performance problems on that particular server. It is therefore encouraged to sign up on another server, such as fairydust.space or tchncs.de. Some projects such as KDE or Mozilla also run their own server (though operated by the same people as matrix.org).
Several TNF members do run their own server, so you can also politely ask them if you can have an account on their server.
Running your own server
You can also run your own server. The reference server is called Synapse and is packaged as chat/matrix-synapse. Just install it and set it up and you can join any of the rooms. The Element Web client is also packaged as chat/element-web, so you can self-host that, too.