I have never been to a jam session where the point of each song was the performance. The point of each song is normally; "here's a song I like to play/sing that I am offering to the circle for participation." This notion shapes my selection of songs. I select songs that I know people can follow easily, and those I believe they would like. This means I take the bad with the good; some players or singers will struggle with the rhythym, some will noodle aimlessly throughout the tune ... but often what happens is magic! Often I hear a collection of creative accompanyment, and warm harmonies, even from the participants who may not be the best players - these are the moments I live for. A jam session is not a performance circle, though, and if performance is the objective of your circle, then you will have to speak up regarding the other people 'playing along' and hurting your performance.