[...] for the complete good is held to be self-sufficient. We do not mean by self-sufficient what suffices for someone by himself, living a solitary life, but what is sufficient also with respect to parents, offspring, a wife, and, in general, one's friends and fellow citizens, since by nature a human being is political. (1.7.1097b7-11)
Here, Aristotle tackles the concept of the greatest good, which is happiness. It's the thing that motivates all human behavior and is, all by itself, perfectly satisfying and sustaining. But Aristotle's no fool; he understands that humans can't find happiness in total solitude because we're social beings. His declaration that a "human being is political" means that we are meant to live in community, interacting with and supporting the common good.
And so, in a word, the characteristics come into being as a result of the activities akin to them. Hence we must make our activities be of a certain quality...It makes no small difference, then, whether one is habituated in this way or that way straight from childhood but a very great difference—or rather the whole difference. (2.1.1103b21-25)
Aristotle speaks here about the necessity to raise a child up in way that will help him develop his "characteristics" (i.e. virtues). He believes that although we may be receptive to such an education from birth, we're not simply born with the virtues up and running. The task of such habituation falls not only to the family, but also to the political community and to lawgivers who enshrine desirable virtues in the laws that they write.
As for private expenditures, there are all those that occur just once (such as a wedding or anything of this sort) and anything the whole city or people of worth take seriously, as well as anything connected with the receiving and sending off of foreign guests or the giving and reciprocating of gifts. For the magnificent person is lavish not on himself but on the common affairs... (4.2.1123a1-5)
All virtues benefit the common good through the actions of good people. In this case, it's the magnificent person (one who spends a lot of his own money to improve the community) who sees to the things that ought to be done to ensure social stability.
Note that even private expenditures (i.e. weddings) have an impact on the community. Aristotle says that even the furnishings in the magnificent person's house have to be excellent, since it's his duty to stimulate the local economy and receive important civic leaders in style.
For he who rules is already in relation to another and within the community. And on account of this same thing too, justice alone of the virtues is held to be another's good, because it relates to another. For it does what is advantageous to another, either to a ruler or to someone who shares in the community. (5.1.1130a2-5)
Justice can only exist in community. Aristotle will make a big deal out of this in his Ethics, especially when he discusses the possibility of committing an injustice to oneself (which he says we can't do). Justice only exists in community because it only exists for people governed by law.
For roughly speaking the majority of the lawful things are those commanded on the basis of the whole of virtue: the law commands us to live in accord with each virtue and forbids us to live in accord with each corruption. Things productive of the whole of virtue are all those legislative acts pertaining to the education to the common good. (5.2.130b23-26)
Aristotle tells us that the law exists not just for the correction of injustice after the fact. It also aims to create the model citizen through an encoding of virtue into law. This "education" can be coercive (note the language: "commands" and "forbids"), which highlights the urgency of conforming to virtuous behavior when living together as a community.
The just in this sense [the corrective] has a different form the previous one, for the just in the distribution of things held in common always accords with the proportion spoken of. And in fact if the distribution comes out of common resources, it will accord with the same ratio that the contributions have toward each other. (5.4.1131b26-28)
It's no surprise that community is at the heart of all conversations of justice in Aristotle's Ethics. If you recall, justice can only exist between people who live together in a society governed by laws. Without this communal life, there would be little opportunity to interact, for better or worse.
For a just society to exist, then, the good belonging to everyone needs to be shared out equitably. But that doesn't mean that everyone deserves to get the same amount. Aristotle proposes the idea of an equitable proportion for all, in which we get back the same proportion of good that we contribute to society. There may be many problems with this concept, but it's at least reasonable on the surface.
For no community comes into existence out of two doctors but rather out of a doctor and a farmer and, in general, out of those who are different and not equal. But these [differing types] must be equalized. (5.5.1133a16-18)
It takes all kinds to make the world go 'round. This isn't a new thought, even in Aristotle's time. He understands that a community is something built on mutual exchange, which cannot exist without a variety of goods and services (i.e. a diversity of people) available to the citizens.
In order for this exchange to work, however, there has to be an equitable way of assessing the value of each thing to be exchanged. Aristotle tells us a little later on that the greatest equalizer of all is money. While some have more and some less, currency allows people in society to get what they need, regardless of what product they have to offer.
Hence all things ought to have a value assigned to them; for in this way there will always be exchange, and if there is exchange, then there will be community. Hence by making these things commensurate, money, just like a measure, equalizes them. For there is no community if there is no exchange, or exchange if there is no equality, or equality if there is no commensurability. (5.5.1133b15-18)
Aristotle defines community essentially as an economic, political unit. In this place, justice begins with exchange. Because exchange within a community makes life possible (because basic life needs must be met), there has to be equality/commensurability between the goods and services exchanged. He doesn't say it here, but Aristotle does talk about (proportional) equality between people in community as an important part of social justice as well. Here, money is the equalizer, keeping need stable. Between human beings, it's a sense of what's just for each person.
But it must not escape our notice that what is being sought is also the just unqualifiedly, that is, the just in the political sense. And this exists among those who share a life in common with a view to being self-sufficient, who are free and equal, either in accord with proportion or arithmetically. As a result, for all those for whom this does not exist, there is nothing politically just in relation to one another, but only something just in a certain sense and by way of a similarity. (5.6.1134a25-29)
Again, Aristotle sees justice tied to human community. But justice isn't a necessity simply because we have to be all up in each other's business: we need it especially to maintain the balance between communal living and what's necessary for an individual to thrive. Unqualified justice (or justice as a general, overarching concept) belongs to the political unit (i.e. the city, or a group of people being ruled by laws), where specific or particular justice exists to maintain equality between citizens.
For in every community, something just seems to exist, and friendship as well. At any rate, people address their shipmates and fellow soldiers as friends, just as those in other communities do. And to the extent that people share in community, there is friendship, since to this extent there is also what is just. The proverb "the things of friends are in common" is correct, since friendship resides in community... (8.9.1159b27-33)
Aristotle makes the argument that the political sphere—the regime—determines the types of friendships can thrive in it. This makes sense, since friendships are a kind of micro-community, a reflection of the larger community around them. The level of justice that exists in society, then, determines what can exist between friends. Friends also must spend their days together, which means that they will live their lives in common, wishing for and doing the same kinds of things—hence, a community of two.