They're not sadly. Noctis is the only playable character (like Sora is in Kingdom Hearts). You can set gambit like orders for the rest of your party but that's about it.
It's one of the game's drawbacks for me personally.
They have said that they ditched Nomura's idea of dividing XV's story through a series of games like XIII and want to tell the whole story in a single game, but I wouldn't be opposed to a game from Luna's point of view, like when you finish KH: Chain of Memories and you get to play as Riku through the other side of the story. I think that would be neat.