Add Readers - Adds readers to a document, reader is a person who can read a document. This action step leaves the readers from the previous state and adds the users listed in this action parameter to them.
The person who was the editor of the app, but is no longer an editor in the next state, automatically becomes a reader in that state. After the Add Readers action step, the editor of the previous state is still the reader of the next state.
Where you can use action:
Readers (text or array of texts) - user email or name of a user group, formula that returns one or an array of user emails or user groups.