There's been a couple times where I've had to subscribe all users to a new newsletter.

I started out with a snippet I found which cleans up the subscriptions, but I lost the link or I would have referenced it here.

Here's the code that will subscribe all users to newsletter node id 16. Note I'm using mysql_insert_id() as a quick fix so you will need to use another method if you're not using mysql.

    $result = db_query("SELECT uid, mail FROM {users} WHERE uid>0");
 
    while ($row = db_fetch_object($result)) {