The reference can pertain to multiple levels:
We might imagine John personally preparing sufficient copies for the seven churches (Revelation 1:11), or perhaps reading his work aloud to a number of scribes tasked with recording what they hear. In that case, the passage amounts to an insistence on accuracy in reproducing the original text.
We might also imagine the reader in the seven churches reading the text to a body of listeners. Then the instruction of the passage amounts to pressing on these listeners the need to listen carefully to every word as read, neither tuning out (so as to omit pieces) nor dreaming (so as perhaps to add pieces).
Clearly, the first possibility is relevant. Yet we note that the opening words of Revelation 22:18 are nearly a verbatim repeat of the words of Revelation 1:3. So the second possibility is distinctly the preferential understanding. Everyone who hears
are first of all the members of the seven churches of Asia Minor—and by extension Christians in the churches around the globe.
18 I warn everyone who hears the words of the prophecy of this book: if anyone adds to them, God will add to him the plagues described in this book,