Unfortunately treewind most festival performers tend to have rehearsed festival or concert "sets" which they will stick to over the course of a number of festivals, you know exactly what THEY will be doing. "Joe Bloggs" the local, on the other hand, who was sitting in the audience and then turns up at a session or sing-around is more like-than-not to come up with material that you as a visitor to those parts may never have come across before.

As to stage performers taking their turn at after concert sing-arounds there is a story concerning the late great Johnny Silvo at one of these sessions where a band consisting of four people used to take their individual turns to perform four songs or sets of tunes from their band's repertoire. During a break at the bar Johnny pointed out to them that the crowd at the session had willingly paid their money and had sat quietly listening to the concert performers all night and that now at this session it was their turn to do their stuff and not an opportunity for the performers to hog the show - They took the point.

By the way if you have to be "invited" to play then it is not really an open session or sing-around. Only experienced one of those once where a well known Glaswegian folk singer managed to turn it into an additional "him and his pals" concert. The rather large room was packed at the start but soon thinned out.