Ding "When Jesus instituted the meal in the first century, did he invite any non-participants to come and watch? No. So why does the Watchtower do so?"" Good question, originally, all the members of the WTS were self-reported anointed. Then as the numbers of attendees approached 95,000 there seemed to be a problem (144,000 total supposed starting from 33 CE!). But then the end did not come in 1925 and the numbers dropped off dramatically the next year. https://www.jwfacts.com/watchtower/1925.php
The Bible is clear that the Great Crowd are in heaven, as was taught by the Watchtower until 1935. (quote of what WTS taught)
"… all the facts and the scriptures bearing upon the matter under consideration show that those who form the great multitude constitute a spirit class, born on the spirit plane." Watchtower 1927 15 Jan pp.19-20 "The Great Multitude," https://www.jwfacts.com/watchtower/great-crowd-other-sheep.php
"Today, according to authentic records, there is a “great crowd” of dedicated, baptized Christians who actively collaborate with the small remnant of spiritual Israelites. Ever since the spring of 1938 they have been invited to attend the annual memorial celebration of Christ’s death, not to partake of the memorial emblems, the unleavened bread and the red wine, but as respectful observers" WT 1979 November 15, p.27