On subscribers section of MailMyCustomer dashboard, you will able to add or delete subscribers. For now, 3 different ways you can add subscribers to your account.
You can paste or type the comma separated email addresses in the textbox located at the top right corner on subscribers section to add subscribers to your account.
All the email addresses in the uploaded CSV file are added as subscribers. You can upload a CSV file on subscribers section.
You can also call a MailMyCustomer API to add subscriber directly from your application. Refer API docs.