Jace was still astonished to see his wife here. How did she get to come to Juliana's party? Did Juliana invite her? But Juliana couldn't have invited her.

Jace was confused, he doesn't think Juliana knows who his wife is nor his wife knows who Juliana is. One thing he found out about his wife when he did his research on her before marriage was that she wasn't exposed to social life and thus knew less or no celebrity. 

She walked in with a brightly tan skin girl that looked about her age. Maybe the girl is her friend and she was the one invited by Juliana and she came...