Of course, when Willow was a ghost she couldn't even do research -- couldn't move pages -- so alternatively, ghosts can only draw Help cards. But they can be given spell cards by another character ("Here, read this"), cast the spells, and "fight" by getting the Help card to do the fighting. ("Go Riley!") Ghosts don't stick in cars, so the movement Help cards don't work for them, and they don't need Joyce, although they can urge her around.
Ghosts can't move during sunlight. Or perhaps during some phases of the moon. Take your pick. Or maybe they can move but not with Help cards. The idea there would be that they can't be seen part of the time.