do add_action on condition

Use has_category() to check if a post belongs to a certain category.

function email_friends( $post_ID )  
{
    if ( has_category( 'uncategorized', $post_ID )
    {
        $friends="[email protected], [email protected]";
        wp_mail( 
            $friends, 
            "sally's blog updated", 
            'I just put something on my blog: ' . get_permalink( $post_ID )
        );
    }
}
add_action('publish_post', 'email_friends');

And you don’t need to return anything for an action handler.