How do the greasers show pride in being in their gang in The Outsiders?

The greasers show pride by standing up for each other.

Pony tells us that greasers stick together.  He is proud of being a greaser, because in his neighborhood it is something to be proud of.  It is more than long, greasy hair, although greasers are proud of their hair.  It is one of the ways they identify each other.  Being a greaser means that you always know someone will be there to look out for you.

When Cherry calls Dally “trash,” Pony defends him.  He tells her that he is also a “grease” and that Dally is his buddy.  Pony tells us that being called greaser by another greaser is acceptable and they don’t mind.  They use the word “playfully” (Ch. 2).  Greasers do sometimes fight each other, but not violently.

Pony tells us that the greasers have different attitudes toward being greasers.

It wasn't fair for the Socs to have everything. We were as good as they were; it wasn't our fault we were greasers. I couldn't just take it or leave it, like Two-Bit, or ignore it and love life anyway, like Sodapop, or harden myself beyond caring, like Dally, or actually enjoy it, like Tim Shepard. (Ch. 3) 

So while all greasers seem to have a fair amount of pride in being greasers, some of them enjoy the lifestyle more than others.  Pony seems to have mixed feelings about the whole thing.  He likes having the gang to look out for him, but he doesn’t like living in fear and being labeled and judged.  He is horrified when Bob calls him "White trash with long hair" (Ch. 4). 

When Pony is preparing for the rumble against the Socs that is supposed to avenge Johnny and Bob, he feels proud to be a greaser.  Generally, greasers show their pride in their gang by fighting the Socs.  This rumble has extra significance because each side has pride on the line, and grief. 

Soda and Steve and I had put on more hair oil than was necessary, but we wanted to show that we were greasers. Tonight we could be proud of it. Greasers may not have much, but they have a rep. That and long hair. (Ch. 9) 

Pony feels that it is sad that all he can be proud of is his hair and his neighborhood, but as he has already established, the pride really comes from the fact that the greasers are a family.  They stand up for each other against the Socs, and against the world.  The greasers are not real criminals, he reflects, like Tim Shephard’s gang.  They are just guys from the neighborhood looking out for each other.