I’ve done it before using WP SSO and auto-adding people to a Mailchimp list when they sign up.
We didn’t explicitly ask for permission to go onto the list but we only sent community related emails with a clear unsubscribe link on each and a note saying they were getting the emails because they signed up to the community.
You can see it in action if you sign up at experts.feverbee.com – you’ll get an automated onboarding series that comes from Mailchimp.