Young Justice was a team consisting of teenage heroes. It's core members over the years were Robin, Wonder Girl, Superboy, Impulse, Arrowette, Secret, Empress, Li'l Lobo, Slobo and The Ray. Red Tornado and Snapper Carr were in mentor positions. The team eventually disbanded, it's core members either retiring or moving on to the Teen Titans.
Stephanie was perhaps the most used teenage hero who was NOT a core member. In fact her frequent use and inclusions in the Secret Origins 80 Page Giant makes one wonder if she was considered for core membership.
Stephanie's first appearance in a Young Justice related book was in JLA: World Without Grownups where she was given a one panel cameo where she saved a boy from drowning in the newly adult-free world. The other one panel cameos were Arrowette and Wonder Girl, who both eventually became core members of YJ.
Stephanie's origin was then retold in the Secret Origins 80 Page Giant, along with the rest of the teams origins (which at that time consisted of Robin, Impulse, Superboy, Arrowette, Wonder Girl and Secret) to the young Greta Hayes by Red Tornado.
Steph's first official encounter with a member of the team besides Robin was not a pleasant one- Greta Hayes (at the time known as Suzie) a.k.a Secret attacked her out of jealousy over her relationship with Robin. The two girls fought it out (Steph acting to defend herself from Greta's rage, Secret on the offensive) until Robin and Red Tornado intervened. Both girls were given a speech about trust.
Stephanie was also involved in the "World Without Young Justice" crossover, where she was Robin in the world that was missing a Tim, Kon and Bart. Her story was told in Robin #101 and when Impulse fixed the world, Steph as Robin was one of the fragments that exploded around him.
Spoiler showed up again in the pages of Young Justice when a multitude of teenage heroes were called to aid the team in avenging the death of Empress's father, Donald Fite, and bringing the Baron of Zandia to justice. Spoiler had speaking parts in Young Justice #50 and Young Justice #51.
In #50 Steph asked Secret if they were "cool", and Secret responded icily she wasn't going to attack Spoiler, if that's what she meant.
In #51 she was seen in multiple panels fighting the villians of Zandia with the rest of "Young Justice Unlimited". She was seen trapped in a pink forcefield bubble with Lagoon Boy and several other heroes. She asked trepedaciously of the approaching Animus "Is...that thing on our side?" and when another team member replied, "That's the Animus. He's with me. Don't worry, he'll have you free in a second." Spoiler said, "Oh good. Just...let him know we're not the chewy center, okay?"
Later in the issue when the team was trying to escape from the pulsing earthen mound that was Lady Zand, Wonder Girl yelled, "Get clear! Get clear!" and Spoiler replied while leaping agilely out of the way, "We're working on it, we're working on it!"
After the ordeal, the team was considering expanding Young Justice to a sort of "YJ Unlimited" with different teams across the globe. Spoiler would have probably been on one of those teams, but unfortunately the team disbanded before they could make the idea more than a possibility.
Spoiler appeared in YJ one last time in their second to last issue when the now totally-unstable Secret went to take revenge on everyone she believed had ever wronged her. Spoiler was on her hitlist, and she sucked Steph into the hellish place between life and death, the abyss, much to Tim's horror.
However, in the next issue Tim talked Secret down, and she expunged every one she'd absorbed with no memory of their terror, obviously including Spoiler.
Stephanie and Young Justice:
Learn more about YJ here