Alter default privileges in postgres for other roles
To add default privileges for a different user, use the FOR ROLE clause:

ALTER DEFAULT PRIVILEGES FOR ROLE user1 ...

See the documentation for details.

As to the required permissions to run that statement, the documentation says:


  You can change default privileges only for objects that will be created by yourself or by roles that you are a member of.


That means that, unless you are a superuser, you can only use roles in FOR ROLE of which you are directly or indirectly a member.