Ginny made the introductions. It was a bit of a funny scene, really. They were all aboard the Hogwarts Express on their way to Hogwarts school, and Cho was walking down the corridor in the company of some of her girlfriends. Neville had just had a rather unpleasant accident with his Mimbulus Mimbletonia, which ended up covering everyone in their compartment in Stinksap, so Harry didn't want to be seen hanging out with either him or Luna. Despite a rocky first encounter, and to Harry's considerable surprise, Luna came through just the same when the Ministry thought he was lying about Voldemort, and he had like three people on his side trying to get everyone else off his back.

Luna was at the time a fourth-year Ravenclaw student, and one of the first people to openly declare their support for Harry in the face of Voldemort's widely-disputed return. So was for that matter Xenophilius, Luna's father and editor of the Quibbler magazine.