Nagios Support Forum

Need to know which hostgroups are associated with each services. Is there a way to find that out through cli ? Or probably run a report on it ?

Here's one way, but it doesn't tell which services have no hostgroup.
grep hostgroup_name /usr/local/nagios/etc/services/* | awk '{print $NF "\t" $1}'

For a count you could do this.
cd /usr/local/nagios/etc/services/
grep -c hostgroup_name * | awk -F : '{print $NF "\t" $1}'

There are probably better, or prettier ways, but I like the shell...

For viewing purposes there's this, via the Hostgroup Overview page (you'll have the same options on the Hostgroup Summary page as well)