The Birds of Prey are a crimefighting team led by ex- Batgirl Barbara Gordon, now the information broker known as Oracle. At the time Steph joined, the only other member was Dinah Lance, the Black Canary, though it eventually expanded to include Huntress, Lady Blackhawk and several others after Stephanie's departure.
Stephanie first became a member of the team when Dinah caught the girl following her. Stephanie pleaded to be allowed to hang out with Dinah and she relented, slyly telling Oracle she once knew another young crimefighter similar to Stephanie.
Stephanie did not go on any documented missions with the Birds of Prey, but she did get Black Canary's help in kicking The Cluemaster and The Riddler out of her house. It's implied she stayed with the team for some time after this, as Tim walked in on her hanging with Babs and Dinah in Robin #99, and in Robin #100 she told Tim she'd been spending a lot of time there and invited him to join them.
Stephanie also left a note with Tim's elevator boy once saying she was "really busy with the Birds, you know, (the human ones)".
Stephanie asked to go on a mission with Black Canary later on, and Canary said she wasn't in the business of putting young people in danger, but that Steph had a lot of talent and should "go back to the Bat Boys".
It is unknown if it was after this Steph left the birds (she wasn't very put out by Dinah's rejection. In fact her reaction was "The Black Canary thinks I have talent. Cool.") but later in Robin #106, Batman mentioned they had fired her, and she never physically appeared in Birds of Prey again.
However, she was not completely out of contact with the members. She called Black Canary in Robin #112 and Canary expressed her condolences about Stephanie's father's death and when Stephanie mentioned she was taking a break from crimefighting, Canary quickly said, "You know, Barbara and I never meant to imply that you should..." and Stephanie said, "No hard feelings."
Her call was to trick Canary into revealing the Riddler's current hideaway by saying she found a note saying the Riddler was going to try something, and then following Canary when she went to check it out. Canary was never aware of this deception, just thinking Spoiler had received a bad tip, and Stephanie confronted The Riddler over her father's death once Canary left the scene to go on another mission.
Stephanie did not go unmentioned in the Birds of Prey book despite her departure. Her picture was on Oracle's screen of possible backup for Dinah on an mission along with others from the Bat Clan in Birds of Prey #56.
She was mentioned after her "death" in BoP too.
Dinah remembers Stephanie when she's trying to save Wildcat's life "It's been a bad year for our community...first poor, sweet Sue...then the endlessly brave and heartbreakingly hopeful Spoiler..." (Birds of Prey #81)
In order to discourage a new teenage heroine, Misfit, from hurting herself trying to fight crime, Barbara shows her pictures from Stephanie Brown's autopsy (Birds of Prey #99)
Stephanie and the Birds of Prey:
Learn more about the BoP here.