Germans.
Possibly citizens of other nations as well. No American Embassy personnel, though, because they were forbidden by their boss at State.
Word has it that the Obama campaign printed the fliers (in German) advertising the event. It was organized by them. They originally wanted the venue to be the Brandenburg Gate, where Kennedy and Reagan both delivered famous speeches, but the German government balked at holding a campaign event at a monument of such historic importance, so Obama's people held it a few miles away at the Victory Column instead.