Great question. You'll want to head to you Audience page and then over to the Segments tab to the button 'create a new segment'.
Create a segment based on signup/manage prefs history, then click 'add'.
Next, where is says 'Search for people who have', you'll choose 'signed up'. You can either specify a date range for those sign-ups, or choose 'at any point'. Then, you will assign which sign up forms were used for these sign ups, and again click 'add'.
Then, click 'done', name your segment and click 'save'. The segment will find everyone who signed up according to the criteria you assigned. Hope that helps! Please let me know if you run into any snags with this. I'm always happy to help.