I believe it's all the talk of the grandness of Pemberley that "caught" Lizzy's attention and her curiosity compels her to join the party at the table in order to hear more. I also think Lizzy to be in the very early stages of developing a sub-conscious interest in Darcy, and that she also can't help being attracted by the idea of such an estate, coming from a not-so-well-off family.