If you have an Email Newsletter campaign with us, we can easily upload contacts into your MailChimp contact list for you. All you need to do is send us an Excel or CSV file of contacts (all we need is first name, last name, and email address).
Don't worry if there are likely duplicates if you send in more than one list - MailChimp automatically cleans duplicates.
If you'd like to learn how to upload contacts yourself, MailChimp has plenty of documentation online that will show you how to do so. You'll just need your MailChimp login (email us at firstname.lastname@example.org if you need it).