Group 'libvirtd' does not exist while installing QEMU-KVM

The issue is about the name of the group. On Ubuntu, the group name should be "libvirt" ...without "d".

so your command should be:

sudo adduser `id -un` libvirt

I had the same problem and for me the solution was to manually create the libvirtd group:

sudo addgroup libvirtd
sudo adduser YOURUSERNAME libvirtd

After that: virt-manager started without telling me to add myself to the libvirtd group anymore.

The group was renamed to libvirt for Ubuntu 16.10 and later. It's in the Ubuntu Server Guide