Integration with MailChimp et al
Most email service providers like MailChimp won't let you send to a list of addresses, even if they're user accounts on your site, without first adding them to a list inside the ESP's control panel.
If you're planning on sending out user emails through a tool like this, it's best to determine the sync strategy before you start signing up users.
That said you have a couple options for existing users:

Create a template to export your user information in CSV format, save it's output and import into MailChimp

Use the Campaign Plugin to send out messages