Kristen was invited because Sean was the one setting it up. And rumor has it, she was the one to get the 33 invite and took the other two.
ok.. but even if Sean set up the Bull and Bear dinner, it was still for Nate's birthday- i'm sure if Nate had wanted to bring a different "date", he would have.
And the same with Kristen's Club 33 invite- i'm sure she chose who she wanted to take along. The second time she went it was her, Nate and her friend Juliet that lives out there. Even though Sean was in CA, too for the 100th Ball.
And the same with the 100th Ball tickets- they could've bought tickets for whoever they wanted to bring along.