Michael didn't tell her not to come. He said "it's not your thing". Of course to you and me, that means don't come, but Brian was sitting there, as was Vic, and they saw how hurt Debbie was by Michael's attitude. So instead of showing up looking respectable, they decided, "if Michael thinks we're an embarassment, then let's be one!" I thought what Debbie said to Michael was right on, because, as we see in 513, he finally got what she meant. Remember? Another congressperson, another instance of being hidden in the back.
And as it turned out, the Senator liked Debbie a lot!
There are times when Debbie is rude and pushy, but this wasn't one of them. Michael was being an asshole.