Actually, they explain it. First, Shinku is there because he's Biscotti's hero, honored guest, and because Yuki wants to spend some time with him. Eclair is there because she and Shinku usually fought together during his last visit. Besides which, she's the head of the Princess's guard knights, and can use the training, and Yuki likes her too. (I think Yuki likes everyone and everything except for demon swords, which she hates with a passion.)

Noir is there because... Galette doesn't have any equivalent of the anti-monster ninja corps that Brioche and Yuki lead. What with the terrible thing that happened at the end of the first series, it's a significant weakness and both Leo and Gaul want to be better prepared the next time something like that happens, if it ever does.

So they've asked Noir to create, and lead, such a formation (presumably based on felines instead of canines). Noir will continue to be part of Genoise, but this will be additional duty for her. Leo asked Millefiore for help in this, and Millefiore has directed Brioche and Yuki to help train Noir to carry this out. That's why Noir was there. It looks like the plan is for Brioche eventually to teach all three of them her special demon-destroying-and-sealing attack.

I wonder if it will turn out that Noir recruits Jaune and Vert for this, too, and make it into a Genoise job.