Neville hugs Ron and Hermione. Harry notices that he's bruised and looks rough. Neville brightly tells Aberforth that people will be Apparating into the bar and should be let through, and then leads the trio into the passage. He explains that all the other passages are sealed and asks if it's true that they broke into Gringotts. Neville says that the Carrows, Amycus and Alecto, are horrible. Amycus teaches students to use the Cruciatus Curse on those who have earned detention, while Alecto teaches in Muggle Studies that Muggles are dirty animals. Neville says he's been standing up to them like Harry used to do, and insists he's not in real danger because he's pureblooded.
That Neville learned the importance of standing up to tyrants from Harry suggests that Harry is more like Dumbledore and more of a teacher in general than he's previously thought. This again shows Harry how to begin to pay Dumbledore's legacy forward and, indeed, how he's already begun to do so by inspiring others to stand up to bigotry and intimidation, just like Dumbledore would've encouraged them to do.
Neville says that they've been communicating through the fake Galleons that they used in Dumbledore's Army. He says that they lost Luna and Ginny, and a few weeks ago, the Death Eaters tried to kidnap Neville's gran to scare him into submission. His gran is now on the run and the Death Eater is in St. Mungo's. Neville leads Harry, Ron, and Hermione into a room filled with about 20 people. It's the Room of Requirement, and has expanded as more students have had to go into hiding. They get food from Aberforth.
The effectiveness and the intricacy of the Hogwarts resistance movement again shows Harry that he's not alone; there are at least 20 people who have been supporting his mission and keeping the resistance alive. This means that Voldemort hasn't yet been able to successfully bring the school under control.
Harry's scar burns and he sees Voldemort discovering the missing ring. He pulls himself back to his reality, gives Ron and Hermione a look, and says they need to get going. Seamus asks what the plan is and Neville asks to help, but Harry refuses. He says that Dumbledore gave them a secret job, which they need to accomplish and then leave. Luna and Dean burst through the passage and Luna is sure that they're here to overthrow Snape and the Carrows. Ginny, Fred, George, Lee Jordan, and Cho Chang come a few moments later. George asks what the plan is and Harry angrily says there isn't one. Ron turns to Harry and quietly says they can help, since even the trio doesn't know what they're looking for. Harry wonders briefly if he's becoming secretive like Dumbledore.
Wondering whether he's becoming like Dumbledore in a negative sense shows that Harry is quickly gaining maturity: he now sees that not everything Dumbledore did was unequivocally good, and he should pay attention to his own behavior and catch himself while he still has the opportunity. That everyone else shows up and offers to help makes it abundantly clear that Harry's community is ready to go and defeat Voldemort; they've been waiting for their leader and now that Harry is here, they're ready.
Harry agrees and tells the room that they're looking for an object, probably from Ravenclaw. Luna suggests Ravenclaw's diadem, but Cho says it's been lost for centuries. Luna notes that there's a bust of Ravenclaw in the Ravenclaw common room wearing the diadem. Harry flits to Voldemort's mind and sees him flying. He agrees to go with Luna to see the diadem. They creep through the castle under the Invisibility Cloak and into the common room. He approaches the bust and studies the diadem, stepping out from under the Cloak. Alecto laughs behind him and touches the snake tattoo on her forearm.
Harry's choice to go with Luna and hear what she has to say about the diadem shows that he's beginning to truly incorporate all that he's learned about pulling information from a variety of sources, even when those sources seem unreliable or unlikely. However, by trusting Luna, he also shows her that he cares about her and, in doing so, likely earns more of her loyalty going forward.