Ron would be both worried and angry. He'd probably want to rush out and start looking for Harry right away. He'd be blaming himself for not being there with Harry on the mission. His friends from the Order of the Phoenix would also get involved. They'd start using their connections and resources to try and figure out what happened. Neville, for example, might use his knowledge of Hogwarts' secrets to see if there are any clues that could lead to Harry. Ginny would be very distraught. She'd be constantly thinking about Harry and trying to come up with her own ways to find him, perhaps using her skills as a Quidditch player to search from the air in some magical way.
Well, his friends would first be in a state of shock. Then they'd quickly spring into action. Hermione would form a plan, probably involving a lot of research and analysis. Ron would be the more impulsive one, ready to jump into danger to find Harry. Luna might offer some rather unique and out - of - the - box ideas, like using her belief in magical creatures that are not well - known to help in the search. The Weasley family as a whole would be very concerned. Mrs. Weasley would be fussing over the situation and trying to keep everyone's spirits up while they search. And all of them together would be determined to find Harry, no matter what it takes.
His friends would be extremely worried. Hermione would immediately start researching every possible spell or charm that could be used to find him. She might also look into any historical records of similar disappearances during Auror missions.