Form question - freeform /mailchimp
Freeform includes mail list integration for MailChimp, Constant Contact (pro only) and Campaign Monitor (pro only).

As per their documentation, form fields can be mapped to your mail list. You can include a checkbox so visitors can choose to subscribe when submitting a form. If they do tick the checkbox, they will be sent a double opt in email (if this is enabled).

Do bear in mind though that MailChimp is rolling out single opt-in from 31st October 2017 unless you're within the EU.