JW's doctrine on the new covenant is one of the most obviously screwed up and perverted doctrines of the JW's.
The scriptures clearly show all Christians partook, as they were all commanded to, and there is not such thing as a 'true Christian' who is not in the new covenant. There is no such thing in scripture as a mere 'observer' of the memorial.
The new covenant is about forgiveness of sins, not about a destination (heaven v earth).
As all Israelites were in the old covenant, even proselytes, so all Christians are in the new covenant.
It's obvious that the great crowd in Revelation must be in the new covenant. They have washed their robes in the blood of the lamb and are even standing in the 'naos' itself before God.