"... it encouraged people to learn the songs before they came to the meetings, so that when their turn came up, they would have something to contribute—preferably a song that the rest of the crew had never heard, if at all possible. The result was that a lot of really good songs began to emerge."

So ~ the exercise, then, was not nercessarily a "sing-along"? It was OK to perform a piece that no one else could be expected to know?

Would one be expected to teach the gang to participate, presumably on an often-repeated chorus? Or not even?

Sounds like song swapping, not necessarily group singing. (Not on every number, anyway). That'd be fine with me, but some gatherings of some groups might have the intention that every participant be able to sing on every selection. In that context, I would imagine that a shared "hymnal" (even a secular one like RUS) would be a basic tool.