You can add contacts manually to your own address book, one by one. Click on the plus sign in the middle column, and choose Create contact.

In the pop-up menu that appears, you can fill in the details of your contact. Make sure to add a valid email address, you don't have to fill the other fields. Click Create when you're done.

You'll immediately have the opportunity to add the contact to (one of) your lists.

You can also add the contact to your lists at a later moment. Click Add to list in the right column when viewing the contact details:

Or select the contact in the middle column and choose Add selection to list in the drop down menu:

One contact can be added to multiple lists, without creating duplicate contacts in your address book. It is possible to create a new list in any of the pop-up menu's.