Can a reader field contains group name ?
If yes, where does Domino check the existance of that group and the members of ?
I didn't find any official documentation about a group inserted into a document reader field, but I saw a reference to it into this response
This document from the designer- help answers your question:
A Readers field may contain Notes® usernames, groups defined in the Domino® Directory provided those groups are of the "Access control list" or "Multipurpose" type, or roles defined in the database Access Control List
But the answer is incomplete. Reader- Fields can also contain Wildcard- entries like */YourOrganization.
And the users don't need to be defined in the domino directory, they can also be in a directory that is known to the server via a Directory Assistance Database as long as the property Trusted for Credentials is enabled for the directories configured there. For groups to be effective the property Group authorization has to be set to yes in additon.