I apologize if this has already been postulated/mentioned, but since the character's name is "Rosalina & Luma" I don't think the Luma needs to be summoned. Like the Ice Climbers, you probably just start with Luma to begin with. It doesn't seem like (from any footage I've seen at least) that Luma can be knocked away/killed. The only time I've seen Rosalina without Luma is during the Up-B special move example, and it's hard to tell whether Rosalina is actually by herself or if the Luma is still on the main stage platform.
Sakurai Mentioned that Luma can be summoned if it is killed in a POTD I think. I want to say it's with Neutral Special as well, but I'd have to look.
Edit: NVM he says, "Pic of the day. A Luma becomes a star. Lumas are tough, but can also be blown away. Luckily, there's an infinite number of them"
Edit some more: It's her Neutral Special. Going from the first trailer. She summons it with that.