It is probably impossible to explain what this is like for people who haven't known this. It's not just a question of knowing a song, or even of group singing the way there used to be "community sings" with published songbooks. It's a question of the memories and resonance of a song, of the relationships you have with with people singing, of the relationships you develop by singing. I travel to the deep south where there is an entirely different set of songs that is generally known and loved. Some of this tradition is dying, but some remains. It's a similar thing.
I sometimes think the closest thing the general culture has is Christmas music. Yeah, I know, people complain and complain about its pervasiveness and ubiquity at this time of year, but it's a culturally common repertoire of the sort that has mostly been lost. My marching band plays for three nights each year in a holiday parade. We play marching but singable arrangements of "Deck the Halls" and "Jingle Bells". You know, the songs everybody complains about (although I think this year I've heard "Sleigh Ride" more times than there are stars in the sky and grains of sand on the shore). But we march down the crowded street, and a large percentage of the crowd gets a very happy look and SHOUTS out the words. Old people, kids, folks of different races. Seriously. It's like a television special or a John and Yoko Christmas 45.
People know these songs. People sing these songs, involuntarily. These songs have powerful resonance, which I suspect is part of the reason people complain so bitterly about them. The sense of communal music is tightly coupled with all the other baggage Christmas carries. But sometimes, in rare moments of forgetfulness and instinct, the pleasure of the music remains.
Even when the music in my head is John Denver and the Muppets.