From eMarketing > Email Lists, open your List then go to the Subscribers tab.

Click the new Quick Add Subscribers button to easily add one or more subscribers simply by entering one or many email addresses, separated by commas.

The emails you enter will be created as Contacts and subscribed to the Email List automatically. 
- Existing contacts are not duplicated
- Existing Subscribers to the list are skipped
- Emails that have unsubscribed cannot be added this way

An alternative method is to enter the details in Contacts then from the Subscription tab in the Contact, tick the Email List to Subscribe to. This approach is more useful when you want to capture a lot of details about the Contact.