Looks like Sarah Hyland
I just watched "Her" and until I actually checked here on IMDB after the movie, I was 100% sure that the girl Samantha got to "play" her (Isabella) was that chick from Modern Family, only with a blonde hair dye. The resemblance was uncanny.
Portia Doubleday:
http://www.imdb.com/media/rm2344410112/nm0234668?ref_=nmmd_md_nxt
Sarah Hyland:
http://www.imdb.com/media/rm3385298688/nm0405103?ref_=nmmd_md_pv