root@master:/etc/salt/master.d# cat /etc/salt/master.d/nodegroups.conf 
nodegroups:
  testgroup: "G@virtual:VirtualBox"

root@master:/etc/salt/master.d# /etc/init.d/salt-master restart
[ ok ] Restarting salt-master (via systemctl): salt-master.service.

root@master:/etc/salt/master.d# salt -N testgroup test.ping
master:
    True
root@master:/etc/salt/master.d# salt -C 'G@virtual:VirtualBox' test.ping
master:
    True
root@master:/etc/salt/master.d# salt -C 'N@testgroup' test.ping
master:
    Minion did not return. [No response]

root@master:/etc/salt/master.d# salt --versions-report
Salt Version:
           Salt: 2019.2.0
 
Dependency Versions:
           cffi: Not Installed
       cherrypy: Not Installed
       dateutil: 2.5.3
      docker-py: Not Installed
          gitdb: 2.0.0
      gitpython: 2.1.1
          ioflo: Not Installed
         Jinja2: 2.9.4
        libgit2: Not Installed
        libnacl: Not Installed
       M2Crypto: Not Installed
           Mako: Not Installed
   msgpack-pure: Not Installed
 msgpack-python: 0.4.8
   mysql-python: Not Installed
      pycparser: Not Installed
       pycrypto: 2.6.1
   pycryptodome: Not Installed
         pygit2: Not Installed
         Python: 2.7.13 (default, Sep 26 2018, 18:42:22)
   python-gnupg: Not Installed
         PyYAML: 3.12
          PyZMQ: 16.0.2
           RAET: Not Installed
          smmap: 2.0.1
        timelib: Not Installed
        Tornado: 4.4.3
            ZMQ: 4.2.1
 
System Versions:
           dist: debian 9.8 
         locale: UTF-8
        machine: x86_64
        release: 4.9.0-8-amd64
         system: Linux
        version: debian 9.8