Welcome to the Dollhouse

Twelve-year-old Dawn Wiener is a shy, unattractive, unpopular seventh grader living in a middle-class suburban community in New Jersey. Her 17-year-old brother Mark is a nerdy high school student who plays clarinet in a garage band and shuns girls in order to prepare for college. Dawn's younger sister, 8-year-old Missy, is a spoiled, manipulative little girl who pesters Dawn and dances around the house in a tutu. Their mother is a shrewd woman who always dotes on Missy and sides with her in disputes with Dawn. Their father is a meek, immature, selfish man who sides with Dawn's mother in arguments with Dawn. Mark is the only one in the family who is even remotely friendly or respectful to Dawn and even that isn't saying a whole lot. Dawn's only friend is an effeminate fifth-grade boy named Ralphy, with whom she shares a dilapidated clubhouse in her backyard.
At school, Dawn is verbally bullied by literally every student she comes into contact with. She is teased and ridiculed and her locker is covered in graffiti. After her teacher unfairly keeps her after school, she is threatened with rape by a bully named Brandon McCarthy, who also has trouble socializing. At home, Dawn's mother punishes her for calling Missy a lesbian and refusing to be nice to her (the same verbal way that other students at the school call Dawn). Dawn is suspended for three days after she accidentally hits a teacher in the eye with a spitball. She refuses to tell her parents or the school principal about the bullying, aware that it will only lead to worse bullying for herself.
Brandon's first attempt to rape Dawn after school fails, but he orders her to meet him again. After she complies, he takes her to an abandoned field. Instead of forcing himself upon her, Brandon starts an earnest conversation with her and kisses her.
Meanwhile, Mark's band is joined by Steve Rodgers, a charismatic and handsome aspiring teenage rock musician who agrees to play in the band in exchange for Mark's help in school. Dawn decides to pursue him romantically after he spends time with her, even though one of Steve's former girlfriends privately tells Dawn she has no chance of being with him.
Dawn and Brandon form an innocent romance, but later in the film, Brandon is arrested during the middle of a class and expelled from school for suspected drug dealing. After getting Brandon's home address from the school secretary, Dawn visits his home and learns that his home life is even worse then hers; he has an abusive and alcoholic widowed father and a mentally challenged older brother who requires constant supervision. After kissing Dawn, Brandon slips out of his bedroom window and runs away to avoid being sent to military school.
After angrily rejecting Ralphy's friendship, Dawn is left with no friends. When she refuses to tear down her clubhouse to make room for her parents' 20th wedding anniversary party, her mother has Mark and Missy destroy it and they punish Dawn by giving Mark and Missy her share of a cake. At the party, Dawn intends to proposition Steve, but gets cold feet and is contemptuously rebuffed. Steve plays with Missy, who pushes Dawn into a kiddie pool. That evening, the family watches a videotape of the party, laughing when Dawn falls into the water. That night, Dawn smashes the tape and briefly brandishes her hammer over Missy as she sleeps. After the party it is mentioned that Steve drop outed of High School and he left for Los Angeles to pursue a music career.
A few weeks later, Dawn's father's car breaks down and her mother has to pick him up from work. Dawn is supposed to tell Missy to find a ride home from ballet class but chooses not to do so after arguing with her; Missy is kidnapped while walking home. When Missy's tutu is found in Times Square, Dawn goes to New York City alone to find her. After a full day and night of searching for Missy, Dawn phones home and Mark tells her that Missy is back home safe for she was found by police after being abducted by a pedophile neighbor who lives on their street. Dawn returns home to find that both of her parents did not notice Dawn's own absence. Later, Dawn's classmates ridicule her as she presents a 'thank you' speech. After the principal finally sticks up for Dawn and tells the unruly students to be quiet, Dawn musters the emotional strength to finish her speech and makes a quick exit from the auditorium.
Summer arrives and Dawn is relieved that her ordeal at school is over for the time-being, having passed seventh grade. Mark tells Dawn that she cannot expect school life to get any better until she starts high school. As the film comes to an end, nothing changes in Dawn's life as her parents continue mistreating and ignoring her, Missy continues to pester Dawn after quickly reverting back to her old ways, and Dawn is still a social outcast in school and the whole neighborhood. Dawn signs herself up to attend a school-sponsored summer camp in Florida to get away from home. In the final scene, on a school bus heading to summer camp in Walt Disney World, Dawn sits among other girls from her school and joins them in singing the school anthem. Unnoticed, her voice slowly trails off as she sits and silently looks out a bus window with a sad look on her face.

Question: Who joins Mark's band?
Answer: Steve Rodgers.