First, select your preferred list where you want to add a subscriber.
1.- Click on 'Manage subscribers' on the drop-down menu and click again on 'Add a subscriber'.
2.- Fill the fields. You’ll need the email at the very least but if you have more information, plug it in.
Note: If you'd want, you can create custom fields of your interest. Please see “How to add custom fields”.
3.- You can continue adding subscribers. When you’re done, go back to 'Lists' to see the contact(s) you just added.
That’s it! You’re done!
Related Article: How do I import multiple subscribers at once?