Q: How do bands do that thing when the singer comes in first (a capella) and the band joins in? Does the singer need to have perfect pitch to that (to be in the right key), or is there a trick to it?
A: haven't you seen barbershop quartets in cartoons, the starting singer pulls out a pitch reference (the one that sounds like a harmonica). regardless, i think you'll find that anyone can remember the key of a song they've heard many times recently, even easier if they've sung it many times recently