To make a kosher grape juice, does one have to be Jewish? or even a bnei noach can make and make it kosher?
Yes, the people involved in the manufacturing of the grape juice, from after it is pressed, have to be Jewish, otherwise it will be stam yanom, and we can not drink it. Although grape juice is not alcoholic, it is still considered wine, since it can ferment into becoming wine. According to R’ Moshe Feinstein zt”l, if the grapes are cooked before they were squeezed, since the grapes were cooked then the juice that comes out of them is already exempt from being stam yanom, otherwise even if the grapes are cooked after they are squeezed it is still stam yanom and not kosher to use.
Rambam Hilchos Ma’acholos Arusor 11-7, Y:D 123-1, 17, !24-6.