This post is mostly about Japan and Japanese, but I hope it will be useful to people looking for ways to expand their ideas on social organization - maybe it can open up some new possibilities for the concepts of self and other in someone's alien group or fantasy society.
The social concept of the "in-group" is quintessentially Japanese, and built into the language in many ways. All their forms of "this" and "that" are based on it. So are their verbs for giving. For example, consider the two non-honorific verbs of giving (used quite commonly to talk about doing favors as well as giving things):
kureru = give to someone in the in-group
ageru = give to anyone not in the in-group
By the way, "kureru" still means "give," because there's an entirely different word for "receive" (morau).
In-group is a concept that has been translated into English quite a bit, so people I talk to have sometimes heard of it, but often they take it to mean the same thing as "in crowd," i.e. a group of socially accepted people. The tough part is, people in an "in crowd" can be part of an "in-group" - but they aren't always.
In-group basically means the people who are members of a social group. Any social group, regardless of its popularity or size. The important part is, the in-group is whatever group the speaker feels associated with at the time that they are speaking. It can change depending on context.
If Mr. Tanaka from Kobe is talking to a non-Japanese person, then he may refer to all Japanese people as the in-group.
If he's in a work-related conversation with someone outside his home company, then all members of his home company become the in-group.
If he's in a work-related conversation inside his home company, but with someone outside his department, then the members of his department become the in-group.
If he's in a conversation at work that concerns his family, then his family is the in-group.
If he's in a conversation within his family, then he himself, alone, is the in-group.
As you may imagine, this can give Japanese language students nightmares. On the other hand, it's a very robust concept in Japan, and on one level, it makes a lot of sense. The in-group is essentially the humbled group in situations where status language is used.
Whenever it's socially appropriate to humble yourself, it's also appropriate to humble all the people that co-occupy your group with you: to humble members of your own company relative to a client company, or to humble members of your department relative to another, or to humble your own family relative to people who don't belong to it.
It's like concentric circles, ripples that can move out from or in toward the individual dropped in the middle.
I hope this can send a few ripples through your ideas of social organization. :-)