This site is an instance of the Shoot messenger, which is a federated websocket-based messenger and group chat (https://github.com/maddyunderstars/shoot)
I tried it out recently and installed my own instance on https://shoot.lehmann.cx/ and made some minor improvements to get the software working with Apache as reverse proxy.
I currently do not have a frontend URL, however it is possible to use the public frontend on shoot.pub with my backend service.
Currently there is no signup if you want to try it out I can send you a registration code, however to get any activity it is probably better if you joined the main domain understars.dev (which also has my main account)