During that time, Dolores Umbridge decided to teach only the theory in her Defense Against the Dark Arts classes for political reasons. Hermione Granger suggested that they found their own group where Harry Potter(who had become quite strong in Defense Against the Dark Arts over the years) would teach all those interested practical Defense Aganist the Dark Arts.
Hermione found "a few people" to meet Harry, Ron, and Hermione in the Hog's Head (the "few people" were actually over twenty). However, Umbridge discovered their meeting, and banned all student organizations. The D.A. afterwards took residence in the Room of Requirement.
However, Cho Chang's friend Marietta Edgecombe betrayed the group to Umbridge. To avoid Harry getting thrown out of school, Albus Dumbledore lied and said he organised the D.A. He managed to escape from the Ministry of Magic.
Though the D.A. ended after this, what the students learned came in handy. Six members of the D.A.(Harry Potter, Ron Weasley, Hermione Granger, Neville Longbottom, Ginny Weasley and Luna Lovegood) were involved the battle in the Department of Mysteries, though only Harry and Neville were left standing by the end, and Neville may have been driven insane had members of the Order of the Phoenix not arrived and Harry was spared because he had a prophecy that the Death Eaters did not want damaged. None of the D.A. members suffered permanent damage in the battle, though Sirius Black was murdered.
Members of Dumbledore's Army: