A friend of mine wants to do a video project of thousands of
   people arguing with each other simultaneously across the planet
   - people paired up randomly where they just start spouting off
   their ranting. Everybody would be in their little cubes just...
   arguing... and I love the concept. It prompted this response
   from me, which you may be interested in, if you're curious about
   what is the most number of people ever to be doing the same
   thing at the same time in the same place?" --- Interestingly,
   you set me on a search. It reminds me of:
   [1]https://www.youtube.com/watch?v=6WhWDCw3Mng where the guy got
   2000 voices singing together, lots of publicity and Ooh and Ahhs
   from the press. The REAL record breaker, you never hear about:
   100,000+ children singing and signing a song every year
   simultaneously in schools across the planet for a charity.
   Sing2sign or sign2sing - I forget - all the videos of it weren't
   nearly as impressive as the 2000 voices production. 100,000
   simultaneous singers is pretty impressive. But then, I was in a
   ROBLOX live stream with my nephew (we watched it because the
   damn livestream was so popular, he couldn't get on the game and
   I couldn't get on to play it with him, and there was over a
   million viewers of the livestream and the chat window was FLYING
   fast. [I think I have my numbers right - I remember being
   impressed at the number]. And I've been in Minecraft
   super-servers with 100,000+ people online in a virtual world
   simultaneously. The cacophony I think would make an excellent
   video project and I, too, would enjoy watching it. It seems as
   if it would be the ultimate antithesis of cooperation but in
   reality, everybody would be perfectly cooperating: They all
   agree to disagree simultaneously.

References

   Visible links
   1. https://www.youtube.com/watch?v=6WhWDCw3Mng