So far we have no idea how Shanks formed the RHP.
imo, he just bought those guys when he was at Mariejois.
It makes a lot of sense imo. Luffy started the SHs by saving Zoro from being executed by Helmeppo/Morgans.
Shanks did the same thing. He saved Ben from slavery by buying him, and maybe other current RHP officers. He then freed them and took them out to sea with him.