In Matthew (26:28), Mark (14:24) and Luke (22:20) when Jesus shares the bread and wine (communion) he says that the wine represents the "blood of the covenant" or the "new covenant". In Luke 22: 28-30 he goes on to say:
However you are the ones that have stuck with me in my trials; and I make a covenant with you, just as my Father has made a covenant with me, for a kingdom, that you may eat and drink at my table, and sit on thrones to judge the twelve tribes of Israel.
In Revelation 5:9,10 it says that the Lamb bought persons for God with his blood "and made them to be a kingdom and priests to our God, and they are to rule as kings over the earth".
So Jehovah's Witnesses believe that the only ones who should share in communion are those who are part of the kingdom covenant, who will be kings and priests and judges with Christ in heaven. They believe the number of a hundred and forty-four thousand in Revelation 7:4 to be literal but the sense in which they are "sons of Israel" to be figurative referring to spiritual Israel, the Christian congregation.
This number of 144,000 is believed to be made up of Christians who have died since the time of the apostles and by now is almost complete. They assume it is not yet complete as the "harm" to the earth and the sea and the trees" (Revelation 7:3) has not yet occurred. Obviously, though, there must be very few left and that is why very few still share in communion at the Kingdom Halls. Whether you partake or not has nothing to do with whether you are an elder, or are even active in preaching. It is simply a matter of conviction, as the scripture says "the spirit bears witness that we are God's children" (Romans 8:16).
Hope this helps.