Pass multiple values in single parameter

VARIADIC

Like @mu provided, VARIADIC is your friend. One more important detail:

You can also call a function using a VARIADIC parameter with an array type directly. Add the key word VARIADIC in the function call:

SELECT * FROM  f_test(VARIADIC '{1, 2, 3}'::int[]);

is equivalent to:

SELECT * FROM  f_test(1, 2, 3);

Other advice

In Postgres 9.1 or later right() with a negative length is faster and simpler to trim leading characters from a string:

right(j.status, -2)

is equivalent to:

substring(j.status, 3, char_length(jobs.status))

You have j."DeleteFlag" as well as j.DeleteFlag (without double quotes) in your query. This is probably incorrect. See:

• PostgreSQL Error: Relation already exists

"DeleteFlag" = '0' indicates another problem. Unlike other RDBMS, Postgres properly supports the boolean data type. If the flag holds boolean data (true / false / NULL) use the boolean type. A character type like text would be inappropriate / inefficient.

Proper function

You don't need PL/pgSQL here. You can use a simpler SQL function:

CREATE OR REPLACE FUNCTION f_test(VARIADIC int[])
  RETURNS TABLE (id int, reference int, job_title text, status text)
  LANGUAGE sql AS
$func$
   SELECT j.id, j.reference, j.job_title
        , ltrim(right(j.status, -2)) AS status
   FROM   company c
   JOIN   job     j USING (id)
   WHERE  c.active
   AND    NOT c.delete_flag
   AND    NOT j.delete_flag
   AND   (j.id = ANY($1) OR '{-1}'::int[] = $1)
   ORDER  BY j.job_title
$func$;

db<>fiddle here
_{Old sqlfiddle}

Don't do strange and horrible things like converting a list of integers to a CSV string, this:

jobTitle('270,378')

is not what you want. You want to say things like this:

jobTitle(270, 378)
jobTitle(array[270, 378])

If you're going to be calling jobTitle by hand then a variadic function would probably be easiest to work with:

create or replace function jobTitle(variadic int[])
returns table (...) as $$
    -- $1 will be an array if integers in here so UNNEST, IN, ANY, ... as needed

Then you can jobTitle(6), jobTitle(6, 11), jobTitle(6, 11, 23, 42), ... as needed.

If you're going to be building the jobTitle arguments in SQL then the explicit-array version would probably be easier to work with:

create or replace function jobTitle(int[])
returns table (...) as $$
    -- $1 will be an array if integers in here so UNNEST, IN, ANY, ... as needed

Then you could jobTitle(array[6]), jobTitle(array[6, 11]), ... as needed and you could use all the usual array operators and functions to build argument lists for jobTitle.

I'll leave the function's internals as an exercise for the reader.