Hogwarts: The Lost Year
Small Acts of Sedition
From Canon we know that Neville was one of the key players to reactivate Dumbledore's Army once it became apparent that it was still needed. I imagine that training akin to what happened in Order of the Phoenix happened but given the nature of what was happening on the school, I would suppose that as well as defensive and offensive spells, a full range of healing (minor injuries) spells were added. Given what the Carrow's liked doing to students (both Cruciatus and physical injury) medical training, magical first aid if you will, would have been necessary or poor Poppy would have been run off her feet.
The reason for reactivating the DA is fairly obvious. Yet again, they have been saddled with a lousy teacher but this time there are no Blood Quills or Snape issues in the class. This time, it's a lot worse. Known Death Eaters, with a passion for hurting students. I can, also, imagine that a couple of Slytherin wouldn't combine their lessons to the classrooms. This prompted Neville to open the classes to slightly younger students this time and tasked the members with watching out for the younger children in the school.
Everything changed following Christmas and Neville, out of fear for his life, had to go into hiding. He changed the way the Room of Requirement acted and created a haven for threatened students. But he didn't stay inside all the time. He sneaked out from time to time, being the thorn in the Death Eater's side and committed acts of sabotage. Everything from an Expelliarmus to a Furnunculus. Neville never went so far as to use an unforgivable on any of the Death Eaters, mainly because with that come memories of his parents' current state, but he has come close. His main reason for crossing the line is that he firmly believes that to cross that line is to start down the slippery slope. For this reason, he intervened in several fights between DA's, Gryffindors and Slytherins. It didn't matter who was getting beaten up, Neville stepped in and ended the dispute.
Safety was always one of the most important things in Neville's mindset and it was for this reason that he had to deny several people membership. People he just couldn't be sure of. But this hasn't stopped him working with them and using the information that they give him. He hopes firmly that one day everyone will be united in one whole, working Wizarding world but he fears that it's about as possible as finding a cure for his parents.
ETA: If anyone has any suggestions for happenings during the final year of Hogwarts or internal to the DA. Please leave me a comment and I'll quite happily talk it over with you. Also, if you want to work out a background between my character and yours (incl. romantic pasts/futures, petty hatreds, etc.) leave me a comment and I'll get back to you. Comments will be screened for now. To keep that air of mystique, you understand!
ETA2: Crossposted to OOC Journal.