You can't find it because it's a neologism. I think it comes from their use of "buzzer" devices, large pushbutton switches that give off a buzz when pressed (actually, technically, the buzzer is the small thing - often a piezo speaker - that creates the buzzing sound, but the meaning has transferred over to those big red switches).
So, "buzzern" means hitting the buzzer button. As to what such a "buzz" means in the context of their show, I don't know. It's probably some form of voting, so "zusammenbuzzern" would mean compiling the list of candidates via buzzer-voting.