Hello,
If you are looking for a way to keep contacts in sync, this is possible with our APIs. We provide methods that can be used to update individual contacts and methods that can be used to upload bulk batches of contacts. You can read more about updating contacts with our API here: http://developer.constantcontact.com/docs/contacts-api/contacts-index.html#update_contact
If you'd like to get started working with our API, you can find more information on how to get up and running here: http://developer.constantcontact.com/get-started.html
As a last note, it is important to mention that using the Constant Contact API will require a developer who is able to build the integration to accomplish this task of synchronizing contacts.
Please let me know if you have any questions!
Sincerely,
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