Neville's looking worse than they've ever seen him, covered in bruises and cuts – he says that Seamus is even worse – but he's delighted to see them. He warns Aberforth that a few more people will be arriving; he's let some other people know that Harry's there.
Harry thanks Aberforth for saving their lives twice, and clambers into the painted tunnel with Neville, Ron, and Hermione. It's an entrance so secret that even the Marauder's Map didn't know about it.
Neville fills them in on the goings-on at Hogwarts as they walk – the Carrow siblings, the two Death Eaters who teach there, have been dominating and punishing students all year long. They've been teaching students things like how to use the unforgivable Cruciatus Curse.
Neville has apparently become quite the rebel, and his cuts and bruises are for talking back to the Carrows.
However, the Death Eaters don't want to spill too much pure Wizard blood, so they haven't been killing students – just torturing them.
The resistance has been communicating via the enchanted fake Galleons that Hermione gave them back in the day, when they were practicing defense spells as Dumbledore's Army (Book 5, Order of the Phoenix).
For a while, Ginny, Luna, and Neville were the leaders of the rehabilitated Dumbledore's Army, but since Luna was taken away at Christmas and Ginny didn't come back to school after Easter, it's just Neville in charge now.
Trying to get Neville to stop his rebellion, the Death Eaters went after his only family, his feisty old grandmother. However, she fought back like a champ and put the Death Eater who came to capture her, Dawlish, in the hospital. She's on the run, but mighty proud of Neville.
Neville figured out that his time at Hogwarts was up – it was time to disappear. But to where?
The answer comes immediately. Ron, Hermione, and Harry emerge from the tunnel with Neville, into a strange and unrecognizable room – it's full of hammocks, bookcases, and tapestries with Gryffindor, Ravenclaw, and Hufflepuff emblems (notably, not the Slytherin serpent).
Though they don't recognize it, it's the Room of Requirement. Neville was being chased by the Carrows, and escaped in to the room – and it knew that he needed a hideout. As other students came seeking escape, the room changed and grew to accommodate them.
Seamus Finnigan, another old friend, excitedly tells them that Neville's got the room completely figured out, and that he knows how to ask it for the right things. The Carrows can't find them there – they don't even know the room exists.
The new arrivals take in their surroundings – there are tons of people there, including their old DA pals Lavender Brown, Padma and Parvati Patil, Terry Boot, Ernie Macmillan, Anthony Goldstein, and Michael Corner.
There's much rejoicing for a moment when Neville tells his army that all the stories about Harry, Ron, and Hermione are true – they did actually break into Hogwarts and got out of Gringotts via dragon.
Before anything else can happen, though, Harry is overwhelmed by pain in his scar – Voldemort is at the Gaunt family shack, and knows that the ring Horcrux is gone. Time is short, and they need to get going to find the Horcrux that's at Hogwarts.
Harry tells Dumbledore's Army that he, Ron, and Hermione have to do something – Dumbledore left them a task, but nobody else can help.
This doesn't go over so well.
Everyone assumed that since Harry has returned, they'll revolt en masse and take over Hogwarts. Luna and Dean arrive (they're the "couple of people" Neville told to come to the Hog's Head), and agree – Dumbledore's Army is Harry's army now, and they're ready to fight for him.
More arrivals show up – Fred, George, and Ginny Weasley, Lee Jordan, and Cho Chang. Everyone wants to fight.
Harry, Ron, and Hermione pause to discuss for a moment; Ron thinks they should accept the help being offered. Harry remembers what Aberforth bitterly said about Albus's secrecy, and decides that he has to open up.
Harry tells everyone that they're seeking something to destroy Voldemort, and that it has to do with Rowena Ravenclaw (all they know about the last unknown Horcrux is that it's something of Ravenclaw's). Luna pipes up and says it might be Ravenclaw's lost diadem. However, the point is, um, that it's lost.
Cho suggests that Harry go up to the Ravenclaw tower to look at the statue of Ravenclaw herself, which depicts the diadem. Luna take him up there, covered in the Invisibility Cloak.
Luna lets the two of them into the Ravenclaw common room (the entrance is not just a password, but a riddle – Ravenclaw really is serious about valuing cleverness!).
The diadem depicted in the statue is a delicate tiara, engraved with Ravenclaw's motto, "Wit Beyond Measure is Man's Greatest Treasure."
A cackling voice behind them jokes that Harry, then, must be broke – it's Alecto Carrow, one of Hogwarts's new professors. She presses her Dark Mark to summon Voldemort.