BuddyPress.org

I don’t think overloading that file is the way to go for the default status change, you can set the default new group status using the bp_get_new_group_status filter, so you could do something like this:

add_filter( 'bp_get_new_group_status', 'venutius_new_group_status_change' );

function venutius_new_group_status_change( $status ) {
    return 'private';
}

I’ve just suggest it, above. To make the default group status private as you asked, you can add that to your bp-custom.php and it should do the trick for you.