My bad, the plugin is better than I expected. (I never RTFM, and noticed the widget after I commented. )
It can subscribe using only the email, no need to be a user in WP
That confusion about subscriber/subscription still remains. As admin it would be ok, add subscriber if there will be a field to add/edit the email address.
Anyway, since it is working, this finesse thing is not an real issue. Especially for a free thing.
Thanks a lot for this plugin.