[…] there are just small groups of friends who stick together, and the warfare is between the social classes. (1.47)
Pony's gang was formed primarily to protect the members from the Socials. There are other Greaser gangs, but they aren't rivals.
Since Mom and Dad were killed in an auto wreck, the three of us only get to stay together only as long as we behave. So Soda and I stay out of trouble as much as we can […]. (1.5)
Loyalty is a big deal for Ponyboy and family loyalty the biggest deal of all. Yet, he and Darrel aren't sure of one another's loyalty for most of the book. What are some things that make them feel disloyal to each other?
You take up for your buddies no matter what they do. When you're in a gang, you stick up for the members. If you don't […] it isn't aging any more. It's a snarling, distrustful, bickering pack. (2.66)
Pony proves his own point at home. When he and Darry start to lose loyalty for one another, their home becomes a place of argument, anger, and mistrust.
"Ponyboy… I mean… if I see you in the hall at school or something and so hi, well it's not personal or anything, but…[…] We couldn't let our parents see us with you all. You're a nice boy and everything…" (3.62)
Ouch! Cherry's comment shows just how committed she is to the rules and norms of her social circles, from peers to parents. It also suggests she's under lots and lots of pressure to be loyal to her group and conform to those rules, even as she wants to break them.
"I had to. They were drowning you Pony. They might have killed you. And they had a blade… They were gonna beat me up." (4.28)
Johnny argues that killing Bob was an act of loyalty to himself, and to Ponyboy. What do you think?
I hadn't realized Johnny was right behind me all the way. (6.45)
When the church is on fire, Johnny wants to save the kids, but he's also motivated by loyalty to Ponyboy, and Ponyboy seems touched and surprised by this.
"That's okay. […] You're a traitor to your own kind and not loyal to us." (8.98)
Ponyboy is speaking out of anger. He's offended that Cherry won't go to see Johnny on account of her loyalty to the memory of Bob. He seems mistrustful of anyone with mixed loyalties or inter-group relationships. Ultimately, Cherry's just trying to make sure nobody on either side gets hurt.