بسم الله الرحمن الرحیم
kum ( کم ) means plural you ( 'men' or 'men & women') kuna (کنَّ) means plural you ( only women)
Yet in conversations it is common to say Alaikum, however if we are to speak with eloquence to an only women group, then say Alaikuna is more correct yet uncommon, especially in non-Arab dialogues.