Amanda Palmer- indie popstar and dark cabaret artiste among other things was famous a few years back for raising a million dollars on kickstarter for a new album. She then toured and in accordance with her usual practice invited local musicians to join her perfomances as volunteers to be paid in fun, beer and hugs. There was a backlash. People not connected to her twitter complained she was exploiting people. She relented and paid 'volunteers' a hundred bucks. But what if she had offered 'playing with Amanda' as a kickstarter option and CHARGED for it. A quick search revealed the band The Polyphonic Spree had done just that- asking $1500 for the priviledge of playing onstage with them. No one had complained, indeed it seemed kind of good sense in these days of hard to monetise pop music...
So as long as money changes hands exploitation has not happened.