Draco invited all the Potters and Weasleys because he cared about the majority of the family. They were important to his children, yes, but many treated Draco kindly as well. He wouldn't repay them by picking and choosing who came to his home. Instead? He left the choice to them. That isn't to say he didn't want Mr. and Mrs. Harry Potter there. It just meant he wasn't going to worry about it. He wanted the night to feel special for Astoria and he also wanted to enjoy it. Hopefully anyone who accepted the invitation felt the same way.
The crowd grew larger so Astoria and Draco separated to mingle and make sure the new arrivals didn't get overlooked. Potter spotted him first, but Draco didn't let on that he felt rattled, if only for a moment. He usually gave Harry and Ginny space, but he also usually felt like the playing field wasn't level. He still didn't feel it was level even as Harry greeted him because he also couldn't help remembering the one and only time Harry Potter had been in his home. Yet another failed moment in Draco's life. But he didn't want to think about that then. He wanted to celebrate what his life was currently, not what it had been. He pushed the memory away and gripped Harry's hand briefly. "Astoria," he replied. "Although I do make an excellent taste tester."