Go to the Contact List tab and access the desired list. Click on the Add Contacts button on the left hand menu.
You will be asked to confirm that you have permission from the recipient(s) to add them to your list. Click on the checkbox and go to the next step.
Select Google Contacts and click on Import data from Google Contacts - enter the account login info as requested.
You will be alerted once the process is complete. Once it is done, you will be able to see your new subscribers under the active subscribers list.