PostgreSQL 11: Replace function body by users from the same group
Consider this example setup:

CREATE ROLE function_creator;

CREATE USER radu;
CREATE USER ion;

GRANT function_creator TO radu, ion; -- this is 'being a member of the owning role'

GRANT ALL ON SCHEMA here_live_the_functions TO function_creator;


Then, when you create functions:

SET ROLE TO function_creator;

CREATE OR REPLACE FUNCTION ...;


After all this, you and ion will be able to change (redefine) the function, which is owned by function_creator.

If you need the debug user, you can add one more level:

CREATE ROLE debugger;

GRANT function_creator TO debugger;

-- and instead of granting function_creator to the actual users, do 
GRANT debugger TO radu, ion;