Frontend User Synchronization
To synchronize fe_users with a list on MailChimp first create a new scheduler task in Typo3. Select the «Synchronize users to MailChimp» task and then select the list you want to use for synchronization. Now save and close the new task. Next, you have to edit the task to configure how the synchronization is done. Select which database fields are used to fill the fields in the list of subscribers.
Usually, you only need to synchronize the users right before you schedule a new campaign. Thus it does not need to be a recurring task. You can trigger the task manually as required.
IMPORTANT: When you execute the synchronization task all subscribers that are not marked as such in the fe_users table will be deleted from the list on MailChimp. So be careful when using existing lists for synchronization.
If you have an existing list with subscribers on MailChimp make sure to execute the «Synchronize users back from MailChimp» Task first. This task will mark all frontend users in Typo3 as newsletter subscriber if they are present in the list on MailChimp. First select the list that should be used for synchronization and then the database field where the email address is stored.
As a last step you would want to extend your registration form, so that your users can choose if they want to subscribe to your newsletter. How you do this differs depending on which extension you are using for doing frontend user registration. Consider the documentation of the corresponding extension. For sr_fe_register you can find the necessary information here http://typo3.org/extension-manuals/sr_feuser_register/3.0.1/view/1/5/. You need to connect the new field with the «subscribed_to_newsletter» field in the database.
If you want to assign more than one group to a user, you can do this by saving a semicolon separated list of groups in the containing database field.