I recently resolved a problem with setting up Netflix. It took multiple contacts and about a week.
It appears that no-one in Comcast customer service fully understands how the X1 Netflix app interacts with your Comcast account and with Netflix.
Here's what I believe to be true.
When Netflix is first added to your package, you get a one time opportunity to associate your entitlement with an email address. If you launch Netflix on your primary X1 box and the end result is a screen with a single "Get Started" button this is your indication that the process is setup and ready to go. Now you have to be careful. If you accidently mistype when entering the desired member name you may be able to watch Netflix in the home via X! but you will not be able to use Netflix elsewhere since account set-up is incomplete. The Comcast agent who ultimately solved my problem was able to delete the invalid registration and get me back to the screen with the "Get Started" button. She pre-populated the app with the yahoo.com email referenced in my Comcast account. Registration with Netflix is then completed by responding to an email from them that establishes a password for your Netflix account. Once this step as complete, you are able to watch Netflix on any device using that username and password.
If you already have Netflix, you should be able to transfer that account to Comcast as part of the "Get Started" process. Comcast may however require the Netflix account email to be "known" to them (ie: referenced in your account as contact info) for that to work. Since Netflix allows you change the member password for an account, changing it to one referenced in your Comcast account before initiating the "Get Started" process may be wise.