I hesitate to ask this question. During some recent research on YouTube I came across a few videos explaining how Watchtower manipulated Revelation 5:10. Revelation 5:10 "You have made them to be a kingdom and priests to serve our God,and they will reign on the earth.”" -NIV
Instead of "on the earth", Watchtower renders the verse "over the earth" and makes the connection of these ones to the 144,000.
So doing some digging into a few commentaries and considering the timing of Rev 5:10 it appears that this group has nothing at all to do with the 144,000. These ones represent the entire body of Christians living on earth today. They are a "kingdom" in that they are the large number. They are "priests" in that they serve God. They "reign on the earth" in that the Christian religion is the largest denomination in the world to date.
Now concerning the 144,000, these ones actually appear to be converts AFTER the Great Tribulation but before Armageddon. So these are ones that decide to serve God instead of getting destroyed when they see all the signs that would be taking place during the tribulation.
So for Watchtower to say that when the tribulation starts that the door is closed for new converts is yet another lie.