I don't know that's my question too!
Dally joined forces with his gang to fight the socs in the rumble at the end of the novel because he has escaped the hospital and he was threating the nurse to let him out of the hospital
"Don't you know a rumble ain't a rumble without me?"
Dallas Winston shows up late to the rumble. He was in the hospital and had to sneak his way out because he didn't want to miss the fight. When he arrives at the rumble, he calls out - which causes Darry to turn his head and Paul punches him, starting the fight.
It is Dally who has joined the rumble. He joined the rumble by "talking the nurse in to let him join the rumble" but he actually used his 10' black switchblade that Two-bit handed back to him at the hospital...
Dally joined the rumble in the book of outsiders
Hch
Dally and pony-boy go see niger right after the rumble.
Dally's late entrance helps them win the rumble
Well, honey, Dally wanted Two-Bit's knife because he probably thought it was shiny and cool, and he's got a bit of a reputation to uphold as the tough guy in the gang. Plus, having a good blade on hand always comes in handy for a greaser like him. It's all about that street cred, darlin'.
it starts talking about it on page 138 but it starts on page 142
the court hearing, the rumble, the loss of both Johnny and Dally and their personal fight with each other (the one where darry and ponyboy try to get soda to take their side).