It is not the secret service's job to decide who gets in and out of a party. It is there job to provide security and to prevent weapons from being in the area etc.
The Salahi's were followed by a Bravo TV crew. Bravo is associated with NBC who has been doing green campaigns and other things to win favor with the administration to make the contracts with GE go smoother.
Tareq Salahi was in the leadership of the American Taskforce on Palestine which Obama associated himself with in his early days (Salahi's name has been scrubbed from the site but Google cache is forever). Rashid Khalidi, who was one of the radicals who helped Obama pay for his college education, was also in the leadership of that organization and then worked with him and the University of Chicago. This means that Obama likely had contact with them when he was a state senator.
Don't blame the secret service. I am nearly convinced that these people thought they were supposed to be there. The administration just doesn't want people doing the homework. Google and Yahoo search is a wonderful thing. I wonder what will happen when the elite media discovers them.