You can already send an email to a list by syncing the list to your Mailchimp account. Now, for most lists, you can send an email directly from the list's results screen, without even connecting a Mailchimp account!
If you have connected your Mailchimp account to your Planning Center account, you'll have the option to send the email directly through Planning Center, or to create a campaign in Mailchimp.
When you send the email through Planning Center directly instead of Mailchimp, the message will appear in the the Communications tab for you and the recipients.
These messages have an unsubscribe link at the bottom. When a recipient clicks that link, they will no longer receive emails sent to any list. You can resubscribe an email address by viewing it on that person's profile.
Because anyone can sign up for a free Planning Center People account, we've added some protections to prevent bad people from sending spam through Planning Center, which would damage the reputation of emails coming from Planning Center and ultimately affecting their ability to make it into people's inboxes instead of spam folders. Besides some smarts on the back end, the only limitation you'll see is that if your list is over 500 people, you'll still need to send your email through Mailchimp.