Attempting to discern "unwritten house rules of etiquette" is too much to ask of newcomers to the music, IMO. It is simply too complex, arcane, and preciously guarded by insiders wishing to maintain the boundaries of their "house session" to be transparent.

It is easy to explain the etiquette to newcomers when one is present. It takes precious little conversation with a newcomer to find out what they do and don't know, what sort of music they play, and what level of experience they are at. Considering that most regular sessions have a recognized leader, there is no reason why that person can't take it upon themselves to greet a newcomer, and make them feel comfortable. Not make them feel at home, but simply welcome and comfortable with a situation that is new for them. If we can't even have the decency to do that for one another, then just what will the long-term outlook be for this music, do you suppose?

There are very few remaining indigenous, homogenous folk music communities left today. Folk music communities are more and more often defined by the common love for the music, not by the fact that your family and neighbors all play the same sort of music you play. That means that each of us is responsible for the future of the music, each one of us is a steward of it for future generations. Not just for our own personal enjoyment here and now.