[Exim] groupmail with exim.

Hi all.
I've searched the web and done the ordinary RTFM but i can't seem to
find any solution to my problem.
I've just setup an Exim server on MacOS 10.3 and it works just fine.
But i want to have a group mail to all users in a domain eg.
group1@??? will send the mail to all users in that group. i
don't need anything fancy , but it would be nice to have something easy
especially when everything else works so great with this server.

TIA
// Fredrik