The best way to learn, it seems to me, is to sit in on the edge of a session where people aren't too worried about anything but having a good time, and play quietly until you've picked up what's going on. For someone who has a fair basic understanding of their instrument, but isn't used to playing with others, it is possible to have a very steep learning curve in that situation. One evening can make an amazing difference.

I suppose the other way is where a bunch of people who are relative novices play together and learn together in a workshop setting, with a couple of more experienced people to encourage and guide them.

The slow jam idea makes sense, I suppose, meaning one where the experienced players are conscious of encouraging and nurturing the new chums - except that I jib at the assumption that slow is easy, and that the more competent you get, the faster you play. In truth it can even be the other way round.