In Charms, Harry and Hermione discuss that Umbridge is reading Harry's letters. She says Filch's Dungbomb accusation was probably a cover story to read his mail. She successfully silences her bullfrog—the spell they’re practicing—and then swaps Ron for his raven. She silences it too. That afternoon, Angelina gleefully tells Harry and Ron that McGonagall stepped in and the Gryffindor Quidditch team can practice again. Ron notices Hermione looking out the window with an unfocused look, and Hermione asks whether they're doing the right thing forming their group. It concerns her that Sirius thinks it's a good idea, as he's restless and probably gets a lot of pleasure from encouraging mayhem at school.
Again, because Hermione functions as a more impartial observer to Harry's train of thought, she's able to see that Sirius isn't an entirely positive and mature influence in Harry's life. While it's true that Sirius is performing a necessary role for Harry by validating his experiences and making him feel cared for, Hermione is right that Sirius is likely trying to live vicariously through them by encouraging them to form dangerous groups like this.
When Harry and Ron head down to the changing rooms that evening for Quidditch practice, the weather is so horrendous that Fred and George are debating whether to use their Skiving Snackboxes to get out of practice. Ron is interested in trying a Fever Fudge, but George tells Ron that he has massive boils between his legs from the fudge. Angelina leads everyone onto the pitch and though she keeps practice going for an hour, nobody can see and it's a waste of time. As Fred and George complain about their boils, Harry's scar suddenly burns, making him jump.
Fred and George's willingness to test their products on themselves in the early stages of development speaks to how much they care about making these products safe and effective for others. This also shows their essentially reckless nature when it comes to their own safety.
Ron hangs back after everyone leaves, and Harry explains that his scar hurt because Voldemort is angry. He's not sure how he knows this, but he knows it's true. Harry continues, saying that Voldemort wants something done faster. At Ron's prodding, Harry thinks back to when his scar hurt in Umbridge's office. Then, he says, Voldemort had been happy. Impressed, Ron says Harry is reading Voldemort's mind, but Harry says he's just feeling Voldemort's extreme moods. He refuses to tell Dumbledore and silently wonders what Voldemort wants to happen. He thinks Voldemort is trying to get at the weapon that the Order is guarding.
Harry continues to fixate on the Order guarding a "weapon," showing that he hasn't entirely internalized the idea that there's more to conducting war than trying to keep dangerous weapons from changing hands. Harry's refusal to tell Dumbledore about what he's feeling means that he won't be able to get help making sense of what's happening to him, thereby making Harry vulnerable to manipulation.
Back in the common room, Harry ignores Ron's concerned looks and tries to concentrate on his Potions essay. After Ron goes to bed, Harry thinks that the Daily Prophet would think he's truly crazy if they knew what was happening to Harry, and wonders what this strange connection to Voldemort even is. Harry falls asleep and dreams that he's walking along a corridor and will finally open the door at the end. He wakes with a start when Dobby taps him—the house elf is by Harry’s bed, and has brought Hedwig. Dobby is wearing what looks like every one of Hermione's elf hats. Happily, Dobby says he's taking all the hats for himself and Winky, as the other elves are insulted by them and now refuse to clean the common room.
Because Harry is aware of how his knowledge of what Voldemort is feeling would come across, he knows that it's even more important to not mention what's happening. Again, this only functions to isolate Harry and make him even less willing to ask for help, as he fears that nobody else will believe him or take him seriously. Though Ron can show Harry that he cares by acting concerned, he's not the best person to have as a sole confidante—he doesn't know any more than Harry does.
Concerned, Dobby asks Harry about his dream and asks if Harry needs help. Harry asks Dobby if he knows of a place where a large group could practice magic in secret, and Dobby tells him about the Room of Requirement. He says it's hidden most of the time, but it becomes whatever a person needs if they walk past thinking of needing something. It sounds perfect. The next day, Harry tells Hermione and though she's skeptical, she agrees to spread the word.
Recognizing that Dobby could have valuable information about the school shows that Harry is learning to expand his community to include others, specifically those who are less powerful than he is. Dobby is one individual who offers Harry a window into the world of beings who aren't valued by Wizards.
At 7:30 that night, using the Marauder's Map, Harry, Ron, and Hermione sneak up to the seventh floor where the Room of Requirement is. The three walk back and forth, thinking of what they need, and a huge door appears. Inside is a room lined with bookcases. Soon, everyone arrives, and Hermione suggests they vote on a leader. They unanimously elect Harry and then decide to call themselves Dumbledore's Army, or the D.A. Harry next introduces Expelliarmus, the Disarming Charm. Zacharias Smith is derisive of learning such a basic spell, but he goes silent when Harry says the charm has saved his life from Voldemort.
While the Room of Requirement shows itself to Harry and the novel's "good guys" in this instance, it's worth noting that the Room would presumably show itself to anyone who needed something, regardless of what House they were in or what their intentions for the room might be. Harry shows that’s learned about the value of mastering the basics when he defends the simple disarming spell. He knows that fancy magic isn’t more likely to work against Voldemort.
Harry divides everyone into pairs and works with Neville. He's glad he decided to start with basics, as many people are struggling. Harry walks around, correcting people, and everyone slowly improves. Cho blushes and makes mistakes when Harry is around. She explains that Marietta is so upset about being a part of the secret group because her mother works at the Ministry. Cho proudly declares her own desire to fight Voldemort after what happened to Cedric, and Luna pipes up to voice her own support for thwarting the Ministry. Soon after, Harry ends the lesson and uses the Marauder's Map to help everyone get back to their common rooms.
Leading Dumbledore's Army allows Harry the opportunity to see his classmates from other Houses as people who can work with him to fight Umbridge, not as adversaries. Again, starting with the basics shows that Harry understands the importance of learning boring and mundane things, as it's only through having a firm grasp of the building blocks that a person can go on to perform more advanced and more exciting magic.