The title can refer to the fact that the two gangs, the Socs and the greasers, are on the edges of society.
The rich Socs "jump greasers and wreck houses and throw beer blasts for kicks, and get editorials in the paper for being a public disgrace one day and an asset to society the next." The poor greasers, conversely, "steal things and drive old souped-up cars and hold up gas stations and have a gang fight once in a while."
There is an irony in the fact that both gangs are outsiders from society and are yet vehement in their dislike of each other: making members of one gang outsiders from the other. Cherry and Ponyboy go some way to bridging the gap, but with very limited success.
Maybe the two different worlds we lived in weren't so different. We saw the same sunset.
Outsiders can also of course refer to the ordinary citizens who do not really touch the gang members, who remain outside the rules and codes of gang warfare. This would include Mr. Wood, the teacher at the Windrixville fire, and Mr. Syme, Ponyboy's English teacher.