I'm not really a believer but can I play?
The JW's assume that the new covenant (Matthew 26:28; Mark 14:24; Luke 22:20) is the same covenant that Jesus concluded with those that stuck with him in his trials. He promised that they would "..sit on thrones and judge the twelve tribes of Israel" (Luke 22:28,29).
This is a lynchpin of the JW two class doctrine. In JW theology, the only purpose in humans going to heaven is to reign with Christ.
Is this a robust assumption? Many Christians would say no. There were certainly other believers at the time (cf. Luke 10:1) who were not invited that night and who would have been actually excluded by the phrase, "YOU are the ones that have struck with me in my trials."