Hi, @snapier3. Thanks very much for this great plugin! If they had a "homage" emoji, I'd use it.

Here's a little feedback. I've installed and configured command and service for five hostgroups. For three of them it is working great! But two HGs, when I run the command from cli or within XI are returning an error:
$ python3 /usr/local/nagios/libexec/check_pctgroup.py -e prd --hostgroup "phoenix_cluster-hg" -c 5 -w 10 -p
Expecting value: line 1 column 1 (char 0)
$ python3 /usr/local/nagios/libexec/check_pctgroup.py -e prd --hostgroup "hive_cluster-hg" -c 5 -w 10 -p
Expecting value: line 1 column 1 (char 0)
So what's different between the HG's? The ones that work have 49, 122 and 146 members. The two failing are much larger. phoenix has 1,414 members, hive has 488. Just spitballing here -- I'm wondering if the check times-out for the larger HG's? I don't have other HGs in between 146 and 488 members, so I don't know what the "break point" is.
I've spot checked a few smaller than 146 HG -- all successful. Checking all of the larger, above 488 members - fail with the same error. I've got 46 HG's so haven't checked them all, but I think I see a trend.
I cloned all services from my first successful one, just changing ARG2 for the HG and a managed host in the HG as appropriate, so I know I didn't fat finger something.
One little niggle with the output that I wonder if it could be tweaked? I think only like one decimal place of % rather than 15 is needed:
OK - Hostgroup FIREBIRD_CLUSTER-HG has 0.684931506849315% of 146 members down. (that's one node down)
Again, thanks for this plugin -- I'm sure other Nagios admins are going to find this useful. Also, the perf data is very useful, because an admin can easily see the waning/waxing of %available over time, which will be informative of events or issues that need to be examined. Not sure yet that even BPI will give this insight. Kudo's!