"Derek." He shook her hand, flashing a brief, short smile. "I, uh, I'm thinking I should get your number so if you can't make it or Cora forgets we can let you know."

It's a reasonable excuse. Derek is not being a creepy asking for her number even though they've just met.

She's cute. He can take a chance on that. Maybe it would get Cora and Laura off his back about socializing.