Access rights management is left to the administrator by way of self-written SQL commands, as is the creation of database users.
This page gives some basic guidelines on how to manage a database with multiple users in a typical scenario:

• One dedicated database user is the owner of the database and all its objects. This is the user that runs the initial creation script.
• One or several roles (=groups) are created with CREATE ROLE commands.
• Access rights are assigned to each role for each database object (tables, functions, sequences, views) with GRANT SELECT,INSERT,… commands.
• Per-user account are created with CREATE USER commands.
• user logins are inserted into the users table. It is not mandatory, but skipping this step will prevent the user interface to trace who does what.
• Users are assigned to groups with GRANT role TO user commands.

Here is a skeleton of an SQL script that grants all rights to the mailusers group. The administrator can use this as a starting point and remove specific access rights to particular groups of users.

-- Tables
GRANT SELECT,INSERT,UPDATE,DELETE ON
 addresses,
 attachment_contents,
 attachments,
 body,
 config,
 files,
 filter_action,
 filter_expr,
 forward_addresses,
 header,
 identities,
 inverted_word_index,
 jobs_queue,
 mail,
 mail_addresses,
 mail_status,
 mail_tags,
 mailboxes,
 mime_types,
 non_indexable_words,
 notes,
 programs,
 raw_mail,
 runtime_info,
 tags,
 tags_words,
 user_queries,
 users,
 words
TO mailusers;
 
-- Sequences
GRANT SELECT,UPDATE ON
 seq_addr_id, seq_mail_id, seq_tag_id, seq_thread_id,seq_attachment_id
TO mailusers;

Special care must be taken with functions since the execute privilege is granted to PUBLIC (every user) by default. In order to restrict the rights to use functions, the administrator should first revoke that privilege:

REVOKE EXECUTE ON FUNCTION
 trash_msg(INT, INT),
 trash_msg_set(INT[], INT),
 untrash_msg(INT, INT),
 delete_msg(INT),
 delete_msg_set(INT[])
FROM public;

It is especially interesting to remove the right to call delete_msg() on all or specific users to prevent accidental deletion of messages when the local policy it to never delete any mail.