My band is now a singer with a guitar/bass/drums trio. She has favorites, so we play those. But most of our book is the result of one of us presenting material to everyone else. If at least two of us are enthusiastic, we'll rehearse a song for a bit and see if it takes. If she's happy and we're happy, it goes on the set list. Whether it stays there depends on whether the crowd reacts positively. It's not exactly democracy, but at least everyone gets to say their piece.