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See How to Use Mailchimp for Salesforce
Group Edition Usage:
We can only create a limited set of reports with MailChimp in Group Edition, as it does not support custom report types. Reports on contacts that have MC subscriber associated to it are possible to create. However, we can not create reports on MC Subscriber that does not have contact associated with it and reports with contacts that do not have MC subscriber.
Except for this limitation, Mailchimp works great in Group Edition.
Setting up Mailchimp and Mailchimp for Salesforce from scratch
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