User's can't get to the login page

my domain is 20now.me.

When I create someone's login/site the form asks for the sub-domain name (i.e. "user" for "user.20now.me"). I enter "user" and they are sent a welcome email saying they can now login to user.20now.me/wp-login.php

I continue setting up their install which means switching their domain name from "user.20now.me" to "user.com".

They get the welcome email and when they click the link they are redirected to 20now.me/wp-signup.php?new=user

Since I don't have public signup there's no way now for them to login.

When I create their account can I just put in their full domain name instead of just their user name? Or is there an easier way to create a full-domain user rather than creating a subdomain for them first?

Or maybe even, is there a way to leave the subdomain active even when they have a full-domain? that way either version of the login links would work.

Thanks!