Alberto (and anyone else)

I've been struggling with this for several days and even started my own thread here about this.

I can only get users from the domain of which the web server is a member to login.

In your post above you say;

More: I can even login using users from an external, trusted, domain! Once users from this domain are given rights to see the portal (put in the right AD groups), they also can logon to DNN.

So how do you give users from an external, trusted, domain rights to see the portal (put in the right Ad groups)?
Please, please, please, be as specific as you can. What AD Groups? Do I need to create the Groups? What are the "rights to see the Portal"?

I feel like I'm very close to making this work, but missing something very fundamental.