If you would like to quickly add a handful of individuals to your list, you can use the Quick Add option:
Navigate to the list you'd like to add the contact to.
Click Quick Add.
On the form, fill in the email and other contact fields.
If the person has explicitly opted in to receive emails from you, check that their profile has granted explicit consent to receive email marketing.
Click Add Contact.
You can also add contacts via Upload
Navigate to Lists & Segments.
Select the list that you want to add contacts to.
Click the Manage List dropdown in the upper right-hand corner.
Click Import contacts.
The import contacts button
Click on ‘Click to Upload’ and select your CSV file of contacts.
Map each column from your CSV to an appropriate contact property.
If a property doesn't yet exist, type in a new property name and click Create New Field.