You can call them sub-flocks or social units, but there really is no such thing as one big happy flock that new-comers will eventually be assimilated into.
The first social unit a chicken experiences is the one with other chicks in the brooder or under a broody hen. This is also the most enduring, lasting the life of the chicken. This is the social unit that gives a chicken their self confidence. Generally, the more chicks in this original unit, the more self confidence a chicken will have. Chicks raised alone or with only one or two other chicks don't have nearly the self confidence that chicks having the benefit of greater numbers. This carries over to how they deal with all future pecking orders. These are the chickens that will have an easier time assimilating into a new flock because they will be much less intimidated by the home chickens.
There are best friend relationships that chickens can form aside from the sub-flocks and regular pecking order, but these usually take years to come about. A lot of times, BFFs are from the brood they were raised with, but not always. I have one original chicken from the very first group of chicks I raised and she's going on eight years. She has formed several BFF relationships over the years, and she's easily the most senior in the pecking order.
This year I introduced two new groups of baby chicks into the flock, and they each adhere to their own sub-flock. Prior to this, the youngest group was three years of age, ranging up to five, six and seven years for the others. I was surprised to see that both groups of chicks were accepted into the flock with almost no bullying by the elders. The only bullying was experienced by the three youngest chicks, who were only four months younger than the first group. This is pretty typical. Also what probably contributed to it was the comparative numbers in each group, five in the first and three in the second, which conferred more self confidence on the first group than the second. Chicks with less self confidence tend to get bullied more.
When you bring home one or two new chickens and introduce them into the flock, it's a double edged sword. What we want is for everyone to get along, but the flock will see them as outsiders and not to be trusted. Also, depending on how much self confidence the newbies have, they will keep their distance from the flock until they feel they've found their place in the pecking order and the home chickens regard them less like intruders. This isn't something that happens quickly.
Chicken World is a complex thing. If we can try to understand it as such, we'll be patient and let the chickens work out their relationships with minimum interference from us. If you can resist micromanaging your flock, you'll find that they will work things out much better than if we try to "help", and in much less time.