Function returning multiple rows?

Hello,

I've been trying to create a function which returns multiple rows, but I
can't get it working. I'm I doing something wrong, or is it not
possible? This is my setup:

create table groups (userid int4, grp int4);

CREATE FUNCTION usersingroup(int4) RETURNS int4 AS 'SELECT userid FROM
groups WHERE grp = $1;' LANGUAGE 'sql';

When doing a

SELECT usersingroup(100);

I only get the first row. Is there any way to get all the rows?

Regards,
Patrik Kudo
kudo(at)partitur(dot)se