Collective Karma of a group of People

I was reading sikh history from internet. In 18th century, the sikhs were spiritually very high, many singhs were jyot vigaassi, had no fear of worldly governments and punished the evil doers.
There was a village where a very arrogant zamindar lived. He raped a poor girl openly in the village. No one in the village helped the girl. When the singhs got to know about it, they came to the village and removed the head of the evil zamindaar and dibanded the entire village.

The villagers who remained silent and watched the rape of the girl were also punished.
This brings a very important concept. We shuld all be trying to do shudbh karam individually, but we shud also make sure that the village, state or country we belong to does not do something terrible wrong with us remaining silent.
The US citizens who protested against the US invasion of Iraq and Afghanistan did a very good noble karam.

On the other hand, majority of hindu population supported the 84 bluestar attack. When Indira was sent to hell, whole of Hindu nation voted her son Rajiv Gandhi into power.
After the Gujarat genocide of muslims, Gujrati hindus voted evil Narinder Modi into power.
By doing so, the hindu nation has earned huge negative karma which they would have to pay.

It's said that in previous yugs, related people too used to incur Karma done by one person but in Kalyug, the rule is that whoever is responsible for a certain Karma, he alone suffers the consequences as is evident from the following Pankiti:

ਸਤਜੁਗੁ ਤ੍ਰੇਤਾ ਦੁਆਪਰੁ ਭਣੀਐ ਕਲਿਜੁਗੁ ਊਤਮੋ ਜੁਗਾ ਮਾਹਿ ॥ ਅਹਿ ਕਰੁ ਕਰੇ ਸੁ ਅਹਿ ਕਰੁ ਪਾਏ ਕੋਈ ਨ ਪਕੜੀਐ ਕਿਸੈ ਥਾਇ ॥੩॥
Satyug, Treta and Duapar are considered to be good Yugs but Kaljug is the best amongst them because the hand that commits the crime that hand only suffers the consequence and no one else is punished in someone else's place.

But when populations or groups of people commit a sin, then they all have to suffer the consequences. If the population supports and condones atrocious acts of their leader, then surely such group of people would get punished collectively. If the whole population of people indulges in sinful activities then surely they collectively get punished as was the case with the people of Aimnabaad who as per prophesy of Siri Guru Nanak Dev jee got punished at the hands of Mughal conqueror Babar.

Kulbir Singh

would the reverse also be true?

for example, in bhai rama singh jee's autobiography he mentions that a sangat of 100 doing naam simran for 1 hour is the same as 1 person doing naam simran for 100 hours.

I am not sure how many times the reward is but definitely the reward is higher when Simran or Kirtan is done in Sangat.

Kulbir Singh