What one concept/ideology is most important in a group of people to encourage them to stop acting as individuals and begin acting in the interest of the whole settlement?
There really isn't one concept that will work for all groups in all times.
It is highly dependent on the society/culture and the settlement age for example. There really is two concepts that are the root of all the others - internal motivation and extrinsic motivation. Pretty much all methods are rooted in fear or desire.
Some methods would be:
- Stringent punishment: This will obviously work much better in a fairly hierarchal society since otherwise punishment is much harder to effectively wield. Moral issues can be brought up by this to great effect.
- Exclusion from the benefits of the settlement unless they are helping the settlement. You can also use this for moral discussion.
- Capitalism: basically a communally created exclusion if there isn't output. Has all the risks and benefits of capitalism in the real world.
- Religious teaching/orders: In a highly religious society orders from religious elders are often akin to law or even greater than law.
This is a tough one, among the reasons why is that nobody has really figured it out in the "real world."
At its most basic, the "usual" methods are:
- Punishment to eradicate antisocial behavior.
- Rewards to encourage prosocial behavior.
These punishments and rewards can be: -physical (prison punishment or monetary reward) -spiritual (hell or heaven) -social (being shunned by neighbors or recognition by the community for work done)
Government, religion and neighborhood block associations are all largely built on these 2 concepts.
Define what their collective enemy is. Every person places themselves into a myriad of different categories at different times to help identify themselves. Different situations require them to define themselves in these different ways. Examples:
- aliens invade earth (definition: human)
- Vikings invade Britain (definition: Saxon)
- Bandits raid village (definition: inhabitant)
If you live in a village in Saxon Britain you are all three of those definitions, but you don't define yourself as Saxon when aliens invade, and defining yourself as human when the Vikings come along won't do you much good because they're human too.
I don't think there is really one idea above all others, but several ways to bind people together. The general idea is oneness, what binds this group together?
Typically it starts out with blood. Family sticks together. That is who you trust and who you work with. As people spread out and meet one another, other ideas emerge. Religion, nationalism, common cause to live/trade/grow/raid. As long as the group has a sense of identity on what makes them them and everyone else not them, then people will start to act in the best interests of the group. Typically following that there will be a written or implicit laws/bonds of honor that govern their interactions with each other. Just start by figuring out what brings them together and gives them a sense of shared identity.
It should be noted that individualism isn't actually the natural state of a human being; it's one of many possible states of culture. The United States has a very individualist culture; we praise and value standing out from the group, being independent, moving away from your parent's home, sticking up for yourself, etc. Japan has a very collectivist culture-- a culture that values homogeneity, listening to your elders/betters/authority figures, multigenerational households, and avoiding conflict. Both forms of culture have both benefits and drawbacks. It might be worth researching various collectivist cultures from around the world to get a good grasp for how it looks in practice.
I'd suggest reading about the key traits of both collectivism and individualism and seeing a list of cultures that fall into various places along the collective/individual spectrum of values.
I wouldn't use it as my only source, but it's a good starting point.