The Weasley family would be heartbroken. Molly would probably cry for days, seeing Harry as one of her own. Ginny, who had a crush on Harry, would be confused and hurt. She would try to talk to Harry and convince him to come back to the right side. Hagrid would be in disbelief. He had always been there for Harry and would find it hard to accept that Harry had joined Voldemort. He might even try to go after Harry on his own, without thinking of the consequences, just to bring him back to Hogwarts.
The Death Eaters would initially be overjoyed. They would think that with Harry on their side, they are invincible. But some of them might also be suspicious of Harry's motives, fearing that he is a double - agent. As for the general wizarding population, they would be terrified. Harry was seen as their hope, and now that he has joined Voldemort, they would feel completely hopeless and defenseless.
The wizarding world would be in shock. People who once saw Harry as their savior would feel betrayed. The Order of the Phoenix would be in disarray, not knowing whether to try to rescue Harry or consider him an enemy. Hogwarts would also be in turmoil, with students and teachers alike not believing that Harry could turn to the dark side.
Harry would probably be in a state of shock and disbelief at first. He'd be filled with a mix of fear, confusion and anger. Voldemort might be initially dismissive or see it as an opportunity to further his own dark plans. Hermione would be practical, trying to find solutions in books and magic. Ron would be more likely to be openly disgusted at first but might come around to support Harry as a friend.
Perhaps Harry joins Voldemort because he believes Voldemort's lies about the wizarding world. Voldemort could tell Harry that the Order of the Phoenix is actually trying to use him and that he would be better off with him. In this scenario, Hogwarts would become a very different place. Teachers would be in a state of panic, not knowing how to deal with Harry's new allegiance. The other students would be terrified, and some might even be swayed to Voldemort's side as well, seeing Harry as a role model. It would be a chaotic and dark situation.
The Hogwarts students would be divided. Some would be in disbelief and would still hope that Harry has a good reason for joining. They might secretly support him. Others, especially those who have lost family members to the Death Eaters, would be furious and see Harry as a traitor. Harry's old enemies at Hogwarts, like Draco Malfoy, would also be confused. Draco might initially think that Harry has finally seen the 'right' side, but as he realizes Harry's true intentions (if they are good), he would be conflicted as well since he's been brought up in a Death Eater - influenced family.
Ron would be extremely angry at first. He might feel betrayed by Harry. But as he realizes that something must be seriously wrong with Harry, he would probably join Hermione in trying to save Harry from Voldemort's influence. They would look for a way to break whatever spell or false belief has hold of Harry.
Hermione would be devastated. She has always been Harry's loyal friend and believed in the fight against the Dark Lord. She would probably try to convince Harry to come back to the right side, reasoning with him about the evil nature of the Dark Lord's actions. Ron, on the other hand, would be both angry and confused. He might initially think it's some sort of joke or that Harry has been tricked.
Voldemort would be calculating. He might be using the Armistice as a way to regain his strength or plot a new strategy. His Death Eaters would be divided. Some might be happy to have a break from the fighting, while others would be eager to continue the war. Dumbledore's followers would be watching closely too. They would be the ones to keep the balance and make sure that the Armistice doesn't turn into a way for Voldemort to take over completely. The Ministry of Magic would be in a difficult position, trying to enforce the Armistice while also dealing with the different factions within the wizarding world.
Hermione would be horrified. She would immediately start trying to figure out a way to rescue Harry. She might start researching in the library for some ancient spells or forgotten magic that could help. Ron would be angry at Dumbledore and would probably want to rush off to save Harry without much of a plan at first.
The Order of the Phoenix would see Harry's joining of the Death Eaters as a major setback. They would likely view it as a sign that Voldemort has become even more powerful. Dumbledore, if he were still alive, would be very concerned and would try to understand what led Harry to this decision. He would also be devising a plan to get Harry out of the clutches of the Death Eaters.