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Personally I think it's better to let users subscribe themselves.
I created a button where they can subscribe.
When you send to many emails, and users mark them as 'spam', you're email will get blocked.
If you want that to happen, you'll probably have to add a template events to the registration hook and make it submit a hidden form in the mail chimp format.
I am a web developer/administrator, specializing in forums. If you have work you need done or are too lazy to do, pm me!