The Newsletter Center is the Memberize bulk email feature that allows you to configure and maintain as many different Subscription Lists as you need. You can allow users to sign up online, have lists that are Members Only, query your database to populate a list, and you can also upload a list of contacts from a third party. To upload your list of contacts browse to the Newsletter Center and select your list or configure a new one. Just below the “Select Members for List” link, that allows you to query your Member Database, there is an “Import” link. This leads to the Import to List page where you can download an Import Template, paste all of your contacts into that template, and then save and upload this to your list.
Like the Database Update, this will append new contacts to the existing list, and will not create duplicate entries (i.e. it will not append a preexisting or duplicate address).