“Church” and “assembly” are synonyms and mean a group of people, not a building. In its broadest sense, as used here, it is the entire body of believers

Colossians 1:18, Ephesians 5:30,

I Corinthians 12:27

Revelation 19:9 “those who are invited” may be the whole church or a subset

Revelation 7:3-17

[sealing the 144,000] literally “these things” which immediately follows the tribe by tribe enumeration of the 144,000