We're All Outside Something
Both S.E. Hinton and her protagonist, Ponyboy, are well aware of the fact that you can be within a group and still exist as an outsider.
Notice how the title refers to multiple people: many different outsiders. If the novel had been title The Outsider, singular, it might suggest that Ponyboy focused on personal isolation— isolation from family, friends, and society at large. Personal isolation is very important to him, but the plural could suggest that he's more concerned with groups of outsiders.
And we definitely see this play out in the novel. Notice how Ponyboy is constantly describing the rules and norms of the different groups—mostly his own gang, other Greaser gangs, and the Socials.
He's also interested in how these different groups interact with each other. Ponyboy's trying to reach out to as many people as possible with his story:
And I decided I could tell people, beginning with my English teacher. (12.71)
Perhaps most, he wants to connect with young men who feel powerless, and who are outside of many opportunities because of economic poverty—the types of boys who might be involved in a gang. In short, kids like himself and his friends.
... But Some Of Us Are More On The Outside Than Others
But back to the original question ("What's Up with the Title?"): the titular outsiders are the main characters, the Curtis brothers and their four friends. We all know what an outsider is, and we've all seen groups of people who are outsiders. In fact, we're all outside of something.
At some point, perhaps on a daily basis, many of us feel separate from and different from others, to some degree or another. We can be isolated by lots of different things, including the politics and economics in our area, our states of mind, our physical conditions, things we've done and things that have been done to us, just to name a few.
But, these things can also connect us. Ponyboy's realization that Greasers and Socials are connected in several important ways, including a shared sense of isolation, is an important insight for him. It is sparked when he meets Cherry, and begins to understand that Greasers and Socs (and everybody else) are connected through nature. He writes, early in the novel,
Maybe the two different worlds we live in weren't so different. We saw the same sunset. (3.18)
Yet, although they are connected, Ponyboy doesn't pretend that this connection really erases the practical differences of their day-to-day lives.
He tells his story in the hopes of creating opportunities for boys like himself, by humanizing them and showing their struggle. By focusing on individuals, he hopes to express some of the extreme problems that face the group, and break down some of those walls that might have seemed to surround such outsiders.