The other comment is correct, but I thought I would just point out a few important differences in the concept of "family" in Japan than in... well probably almost everywhere else.
Legally, "family" means that you are listed on the koseki. As far as I know, every Japanese citizen must be listed in a koseki. Your family name is dependent upon the koseki. When I got married to my Japanese wife, she decided to use my name and since I am a foreigner we had to create a new koseki for our family. I could just have easily have changed my name to her family name and joined her family's koseki. I didn't read the linked article, but I suspect that is what they are referring to as "adoption".
I suppose it is adoption in a way since the person is joining the family, but when you get married one person or the other has to move to the other koseki. Usually the woman moves, but there are many reasons why a man might be invited to join the woman's family instead. The obvious one is that the family has no male children and therefore nobody to continue the family (if all the female children get married and join other koseki). Other reasons can include things like the man being an orphan and wanting to join the other family (this is the case of a friend of mine).
Note that gay and lesbian couples have hacked the koseki to have a form of marriage. One of them simply "adopts" the other. Then they are in the same koseki and legally in the same family. I don't know the details, but I don't think this affords all the benefits of marriage, but I think it gets you quite close.
The koseki is quite strange when you first get exposed to it, but it is really a simple legal instrument. There are definite problems, but for the vast majority of Japanese people it works quite well. The main problems are related to non-citizens. For example, in my case if I were to divorce from my wife, I would revert to a state where I was not on any koseki. This would essentially remove a whole raft of rights that I have when I am on a koseki. You basically aren't a legal person unless you are on a koseki. If I had children, for instance, if I got divorced my wife would automatically get custody because I would not legally be able to take custody, not being a person. There are apparently some reforms coming down the pipe to handle these corner cases, but like I said the system works quite well for the vast majority of Japanese citizens.
> if I got divorced my wife would automatically get custody because I would not legally be able to take custody, not being a person.
This isn't true. You're still considered a person but you're not considered a citizen. Big difference. Koseki does not (nor should it) grant you the same rights as a naturalized citizen of Japan. Becoming a naturalized citizen (through citizinship) is a step which you (or anyone else who qualifies) can take if you want, but it takes a bit of time (same length as becoming a citizen in the U.S, ~10 years).
In your comment, you don't differentiate being a naturalized citizen of Japan and a koseki. But there's a huge difference. A koseki is something different and apart from becoming a naturalized citizen. It shouldn't grant you the same rights if you're a foreigner, it wasn't meant too.
You are correct :-) I was careless in the way I wrote. I did not mean to imply that being on the koseki imparted citizenship. It does not, and you are correct to point out that there is a big difference. What I meant is than in many areas of the law, you are literally not a legal person if you are not on the koseki. They just have no way of dealing with you. Family law is the most obvious place where this occurs, but there are some others a well. Basically, if the law is written to look at the juuminhyou (essentially your registered address), then as a foreigner you are good to go. If it is writen to look at the koseki, unless you are married, you are going to hit a brick wall. Unless, the law has changed very recently (and I know that they were planning to change it), the example I gave (custody in divorce) is a classic example. There is no way that a Japanese judge can grant custody of children to someone who is not on a koseki somewhere because it is legally impossible to do so. Where I was wrong was to say that this is a right. It is not a right (and as a foreigner living in any country you have to get used to the fact that you have a lot less rights than a citizen). It is a consequence of the way the law is written. Still, I can tell you that the difference between when I was living in Japan before I had my name on that piece of paper and after is quite large. Even better is to start a corporation as you then have an entity that you control that is recognized as a legal person (houjin). But that is way off topic...
My understanding is that every Japanese adult has their own Koseki Touhon. So I don't think you join the Koseki, but my understanding maybe incorrect. Children are registered on their parents Koseki. Marriages are also registered. When a name change occurs, this is also registered on the Koseki, but adult individuals retain their own Koseki's.
Foreigners can only be "noted" in a Koseki, they can't have their own, or be registered in, a Koseki.
This is my understanding, but I find the whole thing a bit confusing and am hoping someone will set me right where I'm incorrect.
An adult can still be in his/her parent's koseki until they marry. Most Japaneses do so.
When you marry, if either you or your spouse already have his/her own koseki, the other will be registered to it. If both are under their respective parents' koseki, then always a new koseki will be created for the couple.
Legally, "family" means that you are listed on the koseki. As far as I know, every Japanese citizen must be listed in a koseki. Your family name is dependent upon the koseki. When I got married to my Japanese wife, she decided to use my name and since I am a foreigner we had to create a new koseki for our family. I could just have easily have changed my name to her family name and joined her family's koseki. I didn't read the linked article, but I suspect that is what they are referring to as "adoption".
I suppose it is adoption in a way since the person is joining the family, but when you get married one person or the other has to move to the other koseki. Usually the woman moves, but there are many reasons why a man might be invited to join the woman's family instead. The obvious one is that the family has no male children and therefore nobody to continue the family (if all the female children get married and join other koseki). Other reasons can include things like the man being an orphan and wanting to join the other family (this is the case of a friend of mine).
Note that gay and lesbian couples have hacked the koseki to have a form of marriage. One of them simply "adopts" the other. Then they are in the same koseki and legally in the same family. I don't know the details, but I don't think this affords all the benefits of marriage, but I think it gets you quite close.
The koseki is quite strange when you first get exposed to it, but it is really a simple legal instrument. There are definite problems, but for the vast majority of Japanese people it works quite well. The main problems are related to non-citizens. For example, in my case if I were to divorce from my wife, I would revert to a state where I was not on any koseki. This would essentially remove a whole raft of rights that I have when I am on a koseki. You basically aren't a legal person unless you are on a koseki. If I had children, for instance, if I got divorced my wife would automatically get custody because I would not legally be able to take custody, not being a person. There are apparently some reforms coming down the pipe to handle these corner cases, but like I said the system works quite well for the vast majority of Japanese citizens.