Do people need to opt in when signing up to autoresponders?

The answer depends on how you are deploying your autoresponder marketing campaign. Recipients would need to opt-in when they sign up for your autoresponder through a signup form. Once users have opted in, your sequential autoresponder will immediately begin to send the recipient messages at the intervals you have scheduled.

If you are applying your autoresponder to an existing list you have already uploaded, you do not need to have those contacts opt-in to that list again.