on a cloud VPS, apt-get upgrade pulled a new version of prometheus-node-exporter and it said the group "prometheus" doesn't exist, so:
Preparing to unpack .../prometheus-node-exporter_0.14.0~git20170523-1_amd64.deb ... Unpacking prometheus-node-exporter (0.14.0~git20170523-1) over (0.14.0~git20170523-0) ... Processing triggers for systemd (215-17+deb8u7) ... Setting up prometheus-node-exporter (0.14.0~git20170523-1) ... chown: invalid group: ‘prometheus:prometheus’ chown: invalid group: ‘prometheus:prometheus’