If you want all of your users to be registered into a specific group automatically, go to your components/com_smf_registration/smf_registration.php file.
beneath the
function saveRegistration( $option )you will see this code:
Quote:
mosMakeHtmlSafe($row);
$row->id = 0;
$row->usertype = '';
$row->gid = $acl->get_group_id('Registered','ARO');
if ($mosConfig_useractivation=="1") {
$row->activation = md5( mosMakePassword() );
$row->block = "1";
change the Registered to whatever group you wish your users to be registered as. A good example would be if you want to allow all of your registered users to submit content, such as Authors. Simply change Registered to Author and it is so!

Hope it helps!