I need create a user only to create others users, but, i don't know how create a grant to this?
Exist a method for this, or i need use GRANT ALL PRIVILEGES
?
You can find the available permissions here: http://www.postgresql.org/docs/9.1/static/sql-createuser.html
I believe the one you are looking for is CREATEUSER
The full command you'd want to execute is:
CREATE USER username WITH CREATEUSER;