The event, organized entirely by friends, was held on the Rockville High School football field.
As people began to gather for the event, some exchanged hugs and cried.
What appeared to be hundreds of students, friends and family members walked onto the field and stood there together, many holding hands.
Some wore homemade T-shirts that said, “#1 On The Field, #1 In Our Hearts,” with Miller’s nickname “Meeshmills” on the back.
Many carried balloons that they used markers to write messages to Michelle on, or they attached notes to the balloons with string.
After a moment of silence, all at once, the balloons were set free in front of the evening sun.
“Life’s never going to be the same for me. My life is torn apart,” Miller’s father, Kevin, told the crowd.
“The fact that all of you came here to show how much you love my daughter means everything to me, and I want to thank you,” he added.
“When I see this outpouring of love it not only warms my heart, but it has helped me heal, and I hope you too have found healing tonight,” said school principal Debra Munk.
During the memorial, Miller’s friends also played some of her favorite songs, including Jesse McCartney’s “Beautiful Soul” and John Mayer’s version of “Free Fallin'”.
“My balloon said, ‘We love you Private Lulu’, because when she was little we used to always call her Lulu. Because she has this wonderful spark,” said Michelle’s grandmother, Alice Miller, after the balloon release.
Alice Miller also spoke about how her granddaughter died.
“A staff sergeant in the program called her and told her he was suicidal, and asked her to come over. And she went over to save his life. And he shot her in the head. And I think she’s a hero,” she said, her voice breaking with emotion.
Police say Michelle Miller was fatally shot and killed by Army Staff Sgt. Adam Anthony Arndt. Their bodies were found Monday morning.
Miller had been looking forward to joining the Army Reserve after graduation, say those who knew her.