Sync all of your users
The MailChimp addon comes with a Sync tool that lets you import all your users into a MailChimp list. Navigate to Users -> All Users then click the "Import to MailChimp" link within the list table.
You'll then be redirected to a page where you can select the user role you wish to subscribe and a list.
The tool will then start the import process. Note this might take a while to complete depending on how many users you've got on your site.