That night after dinner, Jane emerges for a few hours. Mr. Bingley is so glad to see her, he hardly pays attention to anybody else.
Miss Bingley tries to keep Mr. Darcy's attention and fails. She pouts about Mr. Bingley's plan to give a ball at Netherfield—balls are so boring when the people who attend are so beneath you—but no dice.
Miss Bingley asks Elizabeth to parade up and down the room with her, since she wants Darcy to check her out and she knows that walking with Lizzy is the best way to do that.
When Miss Bingley asks Mr. Darcy to join them in walking up and down the room, he refuses, saying that it would ruin her reason for walking back and forth.
In other words, she is either sharing gossipy secrets with Elizabeth, or they realize that their figures look best when they are walking about and are trying to get attention. How could he possibly notice their fine figures if he joined them?
(And before you roll your eyes at thinking anyone could notice anyone's figures in those dresses, Regency dresses could actually be pretty form-fitting.)
Miss Bingley's strategy backfires when it turns into yet another opportunity for Darcy and Elizabeth to hate-flirt.