By integrating MailChimp with Reach, your contacts are automatically updated in MailChimp.
To add your MailChimp API Key, from your Reach App Dashboard, go to Settings > Base Settings:
From Here, select the SEO and Social Media tab:
From here you can scroll down the page to see the MailChimp setup:
You must create your API Key in your MailChimp account. To create an API Key with MailChimp, follow their step-by-step instructions listed Here.