"Oh!" She smiles nervously because she doesn't mind being invited to things, it's quite nice, actually, to be acknowledged for once, but she isn't used to such sudden offers.

Especially from people she's just met. Maybe she's just overreacting to a nice man's attempt to be welcoming. She shouldn't overanalyse this. "It would be very nice to go then, sir. It'll be a great deal of help to me and I'll be in your favour in return then for your kindness!"