Nagios Config: Host Groups

/etc/nagios/hostgroup.cfg

/etc/nagios/hostgroup.cfg)
define hostgroup{
        hostgroup_name  fionavar
        alias           Daniel's Computers
        contact_groups  nagios-admins
        members         mornir,darien,revor

Note that you need a host definition for every host to monitor, and every host needs to be part of some hostgroup.