A contact page is automatically created each time you email a new recipient. You can also manually create a Contact page for anyone that isn't automatically tagged as a Contact. In the app site, you can click "New Contact" on the top right of the Contacts page.
The only required field is "Email" but you can also add Name, Company, Title, Phone Number, and Notes. Once you fill out the information you'd like to add you can click Create to save the Contact. Once created, you can go in an update the information later.