Dumbledore's Army 

When Professor Umbridge refused to allow her students to actually perform any magic but merely to read the theory behind it, Hermione came up with the idea of asking Harry to teach Defence Against the Dark Arts to a group of willing students outside of class. He agreed somewhat reluctantly and the group held an initial meeting on October 5th at the Hog's Head tavern in Hogsmeade. After that, Dobby helped Harry find a room called the Room of Requirement and the group began to hold regular training sessions. They decided to call themselves the "DA," which initially stood for Defence Association but which quickly was changed to Dumbledore's Army.

The members of the group signed a parchment which, unbeknownst to them, was jinxed. If any of them revealed what was happening, they would suffer a serious attack of pimples on their faces. In order to alert the members to an upcoming meeting, each person was given a fake Galleon. The serial numbers of the fake Galleon would change to show the date and time of the upcoming session and the Galleon would become warm.

Over the course of the year, the members of the DA learned a variety of spells, jinxes, and charms, including the simple ones like Expelliarmus, and some very difficult ones, like the Patronus Charm. Even Neville and the Creevey brothers became quite accomplished, thanks to patient, careful teaching by Harry.

Meetings ended in April when Marietta Edgecombe betrayed them to Umbridge and they were raided by Umbridge's Inquisitorial Squad. The discovery of the DA eventually led to the dismissal of Albus Dumbledore as Headmaster of Hogwarts (OP27).

In June, when several members of the DA accompanied Harry, Ron, and Hermione to London on thestrals to attempt to rescue Sirius Black, the found themselves engaged in a pitched battle with a group of Death Eaters in the Department of Mysteries. They fought bravely and well, suffering a number of injuries while defending Harry. None of the group was killed, thankfully, due to the timely arrival of members of the Order of the Phoenix.

Members of Dumbledore's Army

1. Abbott, Hannah
2. Brown, Lavender
3. Bell, Katie
4. Bones, Susan
   Had heard about Harry's Patronus from her Aunt, Amelia Bones. 
5. Boot, Terry
6. Chang, Cho
   Cho produced a full Patronus, a silver swan (OP27).
7. Corner, Michael
   Only came in the first place because he was going out with Ginny. Complimented Harry on his performance in the First Task of the Triwizard Tournament.
8. Creevey, Colin
9. Creevey, Dennis
   Who as a second-year student should not have been able to go to Hogsmeade and visit the Hog's Head, much less do the spells! 
10. Edgecombe, Marietta
    Turned traitor and reported the group to Umbridge and suffered for it. 
11. Finch-Fletchley, Justin
12. Finnigan, Seamus
    Joined later.
13. Goldstein, Anthony
14. Granger, Hermione
    Organized things and created the clever charms and jinxes which protected them. At the last meeting, she produced a full Patronus, an otter. Wounded in the Battle of the Department of Mysteries. 
15. Johnson, Angelina
16. Jordan, Lee
17. Longbottom, Neville
    Suffered a broken wand and a broken nose in the Battle of the Department of Mysteries, but was the only student left standing besides Harry. 
18. Lovegood, Luna
    Fought in the Battle of the Department of Mysteries, the only non-Gryffindor D.A. member to do so.
19. Macmillan, Ernie
    Considered this to be the most important thing they were doing this year, not excepting O.W.L.s.
20. Patil, Padma
21. Patil, Parvati
22. Potter, Harry
    Served as the leader and the teacher for the group. 
23. Smith, Zacharias
    Initially reluctant, but he came around. 
24. Spinnet, Alicia
25. Thomas, Dean
26. Weasley, Fred
27. Weasley, George
28. Weasley, Ginny
    Wounded in the Battle of the Department of Mysteries. 
29. Weasley, Ron
    Wounded in the Battle of the Department of Mysteries. 

Members of Dumbledore's Army are indicated on the Lexicon Wizards A-Z list with this icon: