I asked a related question, are we becoming more hive-like?
Even the best evidence of group selection is disputed. Although eusociality is quite well accepted. This seems to be the basis of hive behaviour.
If you think about ants defending the nest, or wasps or bees, an individual experiences threat or harm, releases pheromones. As the amount of those increases the agitation, aggressiveness, and willingness of members to commit suicide increases. So, the defence is founded in experiences of individuals. The response is mediated through the group, as to how other individuals respond as individuals.
Beekeepers wear white suits because bees respond to different colours, it is thought most strongly to black because if a bear attacks the hive they go for the eyes. In Japan honeybees have developed a tactic against the large aggressive hornets that prey on them, of surrounding them in a ball until they overheat. Devil's garden ants and leafcutter ants have developed complex mutualisms with plants and fungi. So there are evolutionary effects acting on the impulses of individuals, which can have their impacts only through a group with the same impulses (even with the bear, it has 2 eyes).
The mouse utopia experiment gives some intriguing though disputed evidence about group behaviour in a mammal. It seems like there was a feedback effect to do with not being able to maintain the degree of inter-connections between mice they had evolved for.
In humans, there have been quite a few people drawing a distinction between cultures founded in herding, where wealth is accumulated slowly & rurally and can be lost suddenly, resulting in honour-based cultures. And cultures founded in agrarian harvests of rice, wheat and so on, with more collectivist ethics and sense that wealth and security derive from collective cooperation, and appeal to authorities.
Philosophically, we have Indra's Net, and in a more modern iteration the peer to peer simulation hypothesis. The Private Language argument, can be said to draw attention to the neccessariness of collective derivation and practice of human intelligence, that we are only individual thinkers at a level of wolf-children, and only develop normally by inheriting collective intelligence embodied in language and in modes of life.
When we look at the emergence of multicellular life, it seems clear that collectives can emerge as stronger than individuals, so it seems inevitable the same pattern will occur in some way. From the engulfing of cells that became organelles and nuclei, to slime moulds where individuals sacrifice their opportunity to reproduce to make spore stalks to aid relatively closely related other cells to spore. In the bacterial biome, the tactics of highly adaptable cells, involve shearing off cell-surface technology from other cells and replicating these, and the jumping of traits between species through the action of viruses and other machinery, eg. as seen with CRISPR. Humans do something similar, instead of evolving claws we make knives and spades, instead of specialised eyes we develop microscopes and telescopes etc. It seems like multicellular organisms had to develop out of that free-for all of evolved technology, and the same may have to happen out of human life - 96% of mammal biomass on Earth is now humans and their farmed prey animals, so our mutualisms have overwhelmingly taken over the planet. An interstellar and maybe even planet-colonising spaceship looks very like the next step, a collecting-together of necessary elements for a complete reproduction of the human-orientated biosphere (you might picture humans as like the first cells with nuclei, with a more sophisticated mode of technology reproduction equivalent to chromosomes).
How much can we know subjectively about our collective intelligence? Probably a lot of it is hidden from us, because it manifests in times of crisis, punctuations in equilibrium. Most of the gene sharing between neanderthals denisovans and humans was about disease resistence (different human blood types are about that too, pandemics have strong selective effects); codified patterns of hygeine may be collective intelligence examples of the same, sewers even more so (Mexico City just before the Spanish was the worlds largest city then, because cholera devasted every other; so this was crucial to the modern city).
Evolutionary biologist Bret Weinstein said something like the end of prosperity is genocide. The quote I can find is:
"Realizing that genocide and war are adaptive is the 1st step to ending them forever".
Germany showed a classic pattern in response to the loss of wealth and power in WW1 and increasing impoverishment afterwards (reparations, great depression etc): turn on minorities, ramp up hostility at borders, strongly enforce group norms and enforcement of submission to the collective. It's easy to see that as all bad, certainly as overwhelmingly destructive and destabilising; but every culture demands different norms for war time, for times of crisis and existential threat, and they will always reccur. It is very likely there are types of individual who struggle in peace-time and thrive in war, because wars are so strongly selective. Unfortunately, the long wage stagnation happening now for all but the elite, resulting from increased automation, is likely spiraling in that direction again. It will perhaps be a crucial test of whether our collective intelligence has increased, when the climate-refugee crisis really hits, a global equivalent of the dust-bowl. So far it doesn't look good.
Pandemics and war are not in themselves collective intelligence. Durkheim identifies the core of religious behaviour as organising around what is sacred. This can be seen as including things like habeus corpus and free speech, which by preventing the state or minority totally subsuming the interests of individuals, prevent (or diminish) the freerider problem which is the main criticism of why pure group selection can't work (so now, like for those spiders linked above, group selection is reformulated as multilevel selection). I would then point to this Durkheimian definition of religious practice, organising around what is sacred set aside and what rules it is taboo to break to the point of stirring a whole community against someone gaining any advantage by violating them, as the primary source of emerging collective intelligence.
I am not saying politics and the military haven't been important too, but as essentially discourses of power I would say they have been far from conscious, whereas religions have to appeal to other modes than just power. The Magna Carta and later English Civil War, and the French & American revolutions can be seen as a further steps into consciously deciding on what to hold sacred, and reorganising societies around that. Enlightenment scientific thought as in some ways further flourishing of at least the evidence-gathering part of this (crucial to making more conscious collective decisions). But we still have politics that is much more tribal than it is evidence based. Perhaps this very discussion is part of understanding what future steps might mean. A single neuron cell, like say a mirror neuron, can perform a complex task, and fire an actuation resulting from it into the collective. Similarly one scientist or philosopher, can apply a special focus or expertise into a message, enhancing the intelligence of the collective, as your neurons do your brain.
As you can guess from my reply, I find this topic especially fascinating. Easily my longest ever post - I hope someone can be bothered to read it! I see this as having the most profound implications for philosophy, and the future of life. Perhaps AI will be the equivalent of white-matter in the brain, and human brain augmentation (like Neuralink) linking us into digital networks as the key to really livening up the neurvous system of conscious collective intelligence. I certainly hope so, it is perhaps the only thing that can save us from ourselves.