Dumbledore's Army

Harry Potter (leader and DA teacher)
Hermione Granger (deputy)
Ronald Weasley

Other members

Hannah Abbott
Lavender Brown
Katie Bell
Susan Bones
Terry Boot
Cho Chang
Michael Corner
Colin Creevey
Dennis Creevey
Marietta Edgecombe (formerly)
Justin Finch-Fletchley
Seamus Finnigan (Last meeting, didn't sign the parchment)
Anthony Goldstein
Angelina Johnson (leaves Hogwarts after Harry's fifth year)
Lee Jordan (leaves Hogwarts in Harry's fifth year)
Neville Longbottom
Luna Lovegood
Ernie Macmillan
Padma Patil
Parvati Patil
Zacharias Smith
Alicia Spinnet (leaves Hogwarts after Harry's fifth year)
Dean Thomas
Fred Weasley (leaves Hogwarts in Harry's fifth year)
George Weasley (leaves Hogwarts in Harry's fifth year)
Ginny Weasley