Where did a sincere Christian living before the advent of Watchtower, had to go after exiting the corrupt Catholic Church? Who was God's organization back then?
That's all you have to ask them. They teach that God had no organization on earth during the centuries after the apostasy started and before C.T. Russell started things going. So they teach that the scenario of God not having a true organization on earth, existed in the past! All you have to do is take that teaching - their own teaching - and extend that scenario to the present. God still doesn't have an organization.
They will counter by pointing to scriptures in Daniel and elsewhere that talk about many roving about and the true knowledge becoming abundant in the time of the end. They will say that the bible prophecies a restoration of true worship in "the time of the end". But guess what: that still doesn't refute your argument because "the time of the end" has not ended yet and they can't tell you when "the time of the end" will end!
So you can simply respond by agreeing with them that there will indeed be a restoration of true worship that the scriptures talk about but is not with the JWs and has not happened yet but is still yet future. They cannot tell you exactly how long the time of the end is and they have no way of demonstrating exactly when during "the time of the end" that true worship is supposed to be established. But you have the JW organization's long, well-documented history of false teachings, lies and false prophecies to prove that they're not the restoration of true worship!