Роль creator 

=> CREATE ROLE creator WITH CREATEDB CREATEROLE;

CREATE ROLE

Роль weak 

=> CREATE ROLE weak WITH LOGIN;

CREATE ROLE

Проверка возможности создания БД 

=> \c - weak

You are now connected to database "postgres" as user "weak".

=> CREATE DATABASE access_roles;

ERROR:  permission denied to create database

Включение в группу 

=> \c - postgres

You are now connected to database "postgres" as user "postgres".

=> GRANT creator TO weak;

GRANT ROLE

Создание БД 

=> \c - weak

You are now connected to database "postgres" as user "weak".

=> SET ROLE creator;

SET

=> CREATE DATABASE access_roles;

CREATE DATABASE