For a few months we have a meetup on lassulus’s server:
Right now we are mostly German speaking participants also we switch to English from time to time.
To join simply run:
$ nix-shell -p mumble --run "mumble mumble://$USER@lassul.us/nixos"
You can also join with the web version.
Join on Android: https://play.google.com/store/apps/details?id=se.lublin.mumla&hl=en&gl=US
Join on iOS: Mumble on the App Store
It will be also announced via IRC on #nixos on hackint.org right before it happens.