I would give them credit for realizing that it would be a bit unusual for a vocalist to sing harmony to an instrument. I am certain that the potential for the need to make certain adjustments has been considered. If they only work from sheet music, I suppose a quick fix would be for certain ones of them to play a third or fifth transposition - if they can do that by sight. However, this doesn't really solve the problem since parallel harmonies quickly become boring. Just go prepared to sing your best and see what happens. The challenge will probably be a healthy thing for all. Good luck!