It's not exactly on topic, but an acquaintance of mine mentioned a program called "Choir choir choir" in Toronto. They just have an open auditorium and people show up and sing in a big choir after getting a little training on different parts to sing. It's really cool. I don't sing in public at all, and even I would consider doing something like this.
Go to youtube and look up "Choir choir choir" and they've got a whole bunch of songs. They usually pick pretty good ones, and it's amazing how good several hundred people can sound together.
The crowd size varies a lot, from 100 or so up to 2,000 who sang "When Doves Cry" after Prince died. My favorite one so far is Bryan Adams' "Summer of '69".