For something like this the level of organization is tiny. I added to tux, and all I need was the picture of tux. I barely needed that because I just refilled pixels people poked into him.

/r/place made me realize that I care about Linux than my country.

Does it make sense that you'd really only need about 300 people, one for every second in 5min? After that, the group can pretty much add squares constantly, one per second. Maybe that wouldn't be fast enough, but it's a good head start on offense, and I'm obvs not accounting for defense.

Organizing groups of people for something as menial as this isn't a hard task. Many subreddits had a pixel template they posted for the content they wanted that made it to those communities front page, and users generated it. Upkeep after that wasn't much of an issue unless they were being raided.