AT STRAWBERRY FESTIVAL this type of thing is called the Band Scramble.
it is hosted by Prairie Flower and Cactus Bob http://www.renwah.com/
who often play with http://www.sourdoughslim.com/
everyone who wants to participate goes to the amphitheatre at noon
and there are hats or bags to put your name in...labelled with instruments...guitar, banjo, etc. and one name from each hat is drawn
and they are given an hour or so to come up with a few songs or tunes,
a joke and a radio spot for the on-site broadcasting "Hog Ranch Radio"
then the bands are issued numbers and the numbers are pulled at random
to decide the order. Voting is done by an applause-o-meter. this is fun.
a theme is chosen, and announced at the beginning, so there is not a lot of time for pre-planning...