I meant the people shown applauding, presumably in the room they have the Tiny Desk in.
Oh. My guess is they saw one final live performance of that song all the way through, of which only the last few seconds plus the audience applause got used in the video. Perhaps they also did a few other songs like a typical Tiny Desk concert? But I haven’t found a recording of that.
More BTS info: Jack Conte (CEO of Patreon) interviews Damian Kulash.
That turned out to focus a lot more on the creative decisions made than I expected, and I was reminded Jack Conte is also a musician and makes music videos (Pomplamoose, Scary Pockets). Clever push to Patreon at the end. The second half of that piece is on Patreon, for free, but you gotta sign up.
I actually saw OK Go live once – they were the opening act for They Might Be Giants. This was back around 2002, when I was in college. No, they did not have any treadmills on stage, as this was a few years before the famous treadmill video, but as best I can remember from a concert from over 20 years ago they did have some fun choreography on stage.
And I just realized I can probably legitimately claim I saw OK Go before they were famous (at least before they were internet famous).
While reading another thread, I wondered if @Robot_Arm has seen this video. 
I would have thought he’d have mentioned that his family was in show business.