I sort of imagine they side-eye each other a lot. Possibly a liiiiittle too similar in some ways. Natalie probably raises an eyebrow at Quinn's attitude, but she can make nice when she needs to. So I'm thinking cordial mexican standoff aha terrible puns I'm sorry
She and Keegan could be "we talk to each other at dueling club" friends. Maybe more, if she ever learns to open up? I don't imagine she and Casey have much to talk about, though, you're right.
Emma and I were discussing that Natalie, Gael and Sibyl could be part of the "oh god why these people why my life" prefect club - if Emma ever gets around to making Gael a prefect. XD Regardless, I imagine she and Sibyl get along pretty well, especially if they've both been on the team for a while.