Obviously, his friends Ron and Hermione would be among the first to join. Neville could also be a key member, as he has shown great courage. Then there might be the members of Dumbledore's Army who are still at Hogwarts. The Weasley twins could also be part of it, using their unique magical inventions to give the army an edge in battle.
In this fanfiction, Harry might start by gathering his most loyal friends like Ron and Hermione. They could then recruit other students who feel oppressed by the Death Eaters' regime at Hogwarts. Harry could use his knowledge of magic, especially defensive spells, to train the army. Maybe they'd also include some magical creatures, like the centaurs or the house - elves who are willing to fight for a better future at Hogwarts.
Hermione would be a great help. Her vast knowledge of magic and books could provide the necessary information for building the planet, like the right spells and magical theories.
Dumbledore might help in a spiritual or advisory way. Even though he's passed away, his wisdom could be passed on through memories or some magical means. Also, Hagrid could be a great help. He could assist in building areas suitable for magical creatures with his love and understanding of them.
In a fanfiction, Harry Potter could be in a situation where he needs to recruit players with strong muscles for the Quidditch team. He might search through Hogwarts to find students who have been secretly training and have the physical strength required for the tough positions in the game, like the Beaters.
Ron could also be a great ally. His loyalty and courage would be important. He might be in charge of security, using his skills in wizard chess to plan out defensive strategies. And of course, the Weasley family in general could offer a lot of support, with their various skills and connections in the wizarding world.
Maybe an old wizard who has been secretly studying the combination of different magical universes for years. This wizard could have made some initial attempts at building a lightsaber - like weapon in the past and can share his experiences and mistakes with Harry. He would teach Harry about the balance between the light and dark side of this new form of magic, just like the Jedi teachings in Star Wars.
Well, he might start by using his charisma. Naruto has always been good at making friends. He could reach out to old allies from his village, like Shikamaru and Choji. Also, he might convince some of the outcast ninja who have been mistreated in other villages to join him. His reputation as a hero who saved the world would surely attract a lot of people.
Well, perhaps Harry uses his magic to shape the planet. He might create unique magical creatures and ecosystems. For example, he could make a forest where the trees are made of living fire that doesn't burn but gives warmth and light.