When you connect a Twitter account to a Broadcaster within your nation, NationBuilder pulls in your followers and interactions on Twitter. Similarly, when you connect a Facebook page to a Broadcaster, when people RSVP to an event in your Facebook page or comment or like a post on the page, that interaction is pulled into your nation. You can mark these people as needing following from Broadcaster > [Name] > Settings > Twitter and Broadcaster > [Name] > Settings > Facebook. You could then reach out to the person and encourage them to interact with an aspect of your website that would encourage them to register with your nation and you can then tag people who have followed you on Twitter or interacted with your Facebook Page. While your interaction with people on other platforms is pulled into your nation, no one is registered without knowing. For someone to sign into your nation with Twitter or Facebook, they must give permission to your nation to connect in that way.

The most efficient way to create a list of people who have signed into your nation from a signup page is to do an Advanced Search. Go to People > Advanced Search. Click on the Origin tab. There is a question "Joined from a page?" If you click yes, that will show you all the people who joined from a signup page. If you check no, that will show you all people who have interacted with you in other means, including people who were imported into your nation. You can combine this with a Name and contact search for following you on Twitter or having Facebook info. These advanced searches can be saved as a filter for easy access to people in your nation base on origin. 

More information on creating a filter is available in this FAQ.

Here is a how-to video on tagging and advanced search.