"Ναι, γεια σας!"
Γεια σας! is used when speaking to several people, or when speaking politely/formally.
Γεια σου! is used when speaking to one person whom you know well.
It's a T/V distinction (like tu/vous in French or ты/вы in Russian or du/Sie in German or tú/Usted in Spanish) as well as a singular/plural distinction in the T form.
(So as in French or Russian, the same form is used for speaking to one person formally, or to several people.)