Import existing contacts
Add contacts manually or import via CSV
Login to your account and go to the subscribers section.
Then click the "Add Followers" button.

You will see two options:
A) add contacts manually, one at a time
or
B) import a CSV file

The structure of the CSV file should be that contact entries begin on row 1.
And:
Column A: First name
Column B: Last name
Column C: email address