If you want to get it really right, then you need to say:

 - To one woman: as-salāmu ʻalayki (or ʻalayk)
 - To two women: as-salāmu ʻalaykumā
 - To three or more women: as-salāmu ʻalaykunna (or ʻalaykunn)
 - To a mixed group (men and women): as-salāmu ʻalaykum