When he heard the crack announcing that people had just appeared, Bill turned to see who it was. At first, he didn't recognize the two people, but that had more to do with his distance from them than actually not knowing them and what they represented. Panic quickly spread through him and he was up out of his chair around a table with other Order members before Alecto had the chance to kick the cake table over. In mere seconds, Bill found his wand, withdrew it from his inner coat pocket and looked around frantically for Fleur.
It put him at ease to see her in very much the same position as him. Bill knew that Fleur could more than take care of herself, but he would definitely keep one eye trained in her direction - ready to interfere if anyone threatened to hurt her. With so many of their family and friends in attendance, the reality of how bad things could get started to set in and Bill glanced around at the other Order members, making sure they were aware of the situation.
They were, from what he could tell.
"What do you want?" he yelled at the pair of Death Eaters.