howto manage groups - hidden from subscribers ?

We need to send newsletters to "hidden groups" - subscribed users dont see and can't subscribe themself to . How could we manage this? Beside we are very happy with E-Newsletter so far. Currently we always creat a new temp group, import all users, send the newsletter and delete the temp goup right away.. But thats not safe, a bit crazy and very uncomfortable to handle. We even mean, consider the bebefit for every WPMU-user that E-Newsletter could handle hidden groups as well ..