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 -> MailChimp Sync to access the tool.
Select the user role you wish to subscribe and a list. It's recommended you also enable the double opt-in option, then press the "Sync users" button.
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.