If you want to subscribe your users to your newsletter during the registration process on your site and if you use the standard joomla registration, then you can use this plugin to show a newsletter checkbox. Not only this, but you have the option to select any field from your subscription form (that you've created on mailchimp.com). In the basic settings select the list that you want your users to be subscribed to, configure which fields should be shown during the registration process & just enable the plugin after that. That is all you have to do. Now when you navigate to your registration page you should see a "newsletter checkbox", once selected the configured fields should show up.
Please note that any required field that you've created on mailchimp.com needs to be selected in the plugin settings. If you don't do this, then users won't be able to subscribe to your newsletter as the required fields won't be shown during the registration process.
There is an exception to this - if you use the field mapping. You most probably don't want to ask the user for his email, first name and last name - as he already entered those in registration form. If you enter the above code in the field mapping, then we will read the email, fname and last name out of the information that the user has provided in the form. This makes the subscription process easier as the users don't have to enter the same information twice.
EMAIL=email FNAME=name:0 LNAME=name:1
If new user account activation is turned off and your users are immediatly registered on your site, then we will also register them for the newsletter, but they will have first to confirm this by clicking the "subscribe me" button in the email that they will receive from Mailchimp.
If new user account activation is set to self (which means that they will receive an activation email), then we will activate their subscription without sending them a second mail. Whenever they click the activation link they will be subscribed to your newsletter as well.
In both cases this makes sure that we follow the double opt-in process.