Every professional band could add strings and horns on a voluntary basis, but they don't because they realise the right thing to do is always pay musicians. Of course, it would be an incredible opportunity of a lifetime to volunteer to play rhythm guitar free of charge at a U2 show, or play sax on stage free with The Rolling Stones, or add your cello playing to McCartney's band. But apparently it's ok now if you are an internet sensation. I'm surprised she didn't run a kickstarter campaign to fund her tour. People could donate, with the prize of being in the band for a night. That way people could pay to play.