Add all users of site to e-newletter

We need a way to email all users on the site easily.
Would be great to use e-newsletter for that if possible.

So is there a way to set up an initial group in e-newsletter consisting of all users on the site and then allow them to opt out.
As it is only about half of the site users show up in the list, as "active users"
Well I need to reach the inactive ones with some info...
Grateful for any help with this!