In my opinion, you could answer this in two ways.
The most obvious answer would be that the Greasers are the outsiders. They are looked down on by society, they are poor, they have few real prospects in life. That's what you would usually think of as an outsider.
I think you could also argue that all the teens in the book are outsiders. Even the Socs, who seem to have it all, are unhappy in some pretty serious ways. You could define outsiders that way too -- people who feel like the society has nothing for them. In that case, all the teens we meet are like that and (if you think about it) all teens are somewhat this way in real life.
In Hinton's book "The Outsiders," the Greasers are the outsiders that exist outside of the socially accepted society. The reader is aware that there is a larger group of them in the story, but become made aware of certain boys in the greaser group that Pony Boy grows up with. Johnny, Two Bit, Steve Randall, Pony Boy, Dallas, Soda Pop, and Darry are in his collective group. However, the greasers consist of boys that are deemed juvenile delinquents from a poor neighborhood that are not viewed as having anything to offer.
The greasers are always on the outside looking inward at the things that the Socs have such as new cars, money, good clothing, and the chance to go to college. They seem to live in a perfect world compared to the greasers.
I can only answer this question as my own opinion since it is not really a factual question.
That being said, I believe the Greasers were meant to be the Outsiders in the novel. When we think of the word "outsider", what comes to mind? Those who do not fit in with the majority and the norm, those that are different. In this novel, the Socs' would have been considered the normal, typical teenagers in the area. They all came from money, had all the nice cars, popular/stylish clothing, good looks and played varsity sports. They were the popular kids in school.
The Greasers were the complete opposites. They had very little money, old and worn clothing, used their two legs as transportation and were mostly looked down upon and ignored, except amongst each other.