Unexpected Return from Function

I have a FUNCTION:

CREATE FUNCTION get_responsible(text)
RETURNS TEXT AS '
DECLARE
responsible_list text;
my_record RECORD;
BEGIN
FOR my_record IN SELECT login FROM employee WHERE id IN (1,2) LOOP
responsible_list := responsible_list || '', '' my_record.login;
END LOOP;
RETURN responsible_list;
END;
' LANGUAGE 'plpgsql';

The employee table is such:

id | login
-------------
1 | anthony
2 | mary
-------------

I expect the SQL statement "SELECT get_responsible('1,2')" to return
something like so:

get_responsible
---------------
anthony, mary
---------------

But instead I receive:

get_responsible
---------------

---------------

If I get rid of the concatenation, like so:

CREATE FUNCTION get_responsible(text)
RETURNS TEXT AS '
DECLARE
responsible_list text;
my_record RECORD;
BEGIN
FOR my_record IN SELECT login FROM employee WHERE id IN (1,2) LOOP
responsible_list := my_record.login;
END LOOP;
RETURN responsible_list;
END;
' LANGUAGE 'plpgsql';

I receive last result (for id = 2), like so:

get_responsible
---------------
mary
---------------

The SELECT statement itself runs fine, so I know it is returning two
records.

Also, the reason I am passing a 'text' datatype to the function, is
because I'd ultimately like to have the "WHERE id IN" statement to be
dynamic, like so:

CREATE FUNCTION get_responsible(text)
RETURNS TEXT AS '
DECLARE
responsible_list text;
my_record RECORD;
BEGIN
FOR my_record IN SELECT login FROM employee WHERE id IN ($1) LOOP
responsible_list := my_record.login;
END LOOP;
RETURN responsible_list;
END;
' LANGUAGE 'plpgsql';

If anyone can help me with this, I'd be much appreciative. I've been
trying combination after combination of things to try and resolve this
for the past 6 and a half hours.

Thanks,

Anthony "pulling his hair out" Bouvier