If you've sent an invitation email out and want to resend it to people that haven't responded (registered or declined) yet, we can filter a new send. Here's how....
1. Go to your event menu, select Communications (formerly Emails & Invitations and then choose Add Scheduled Email
2. Set your reminder up with campaign name, subject, content etc.
3. Select to send to All Contacts. Then you can utilise the system tags (see the article on them here). Select to send to everyone you've invited so far, so the tag for that is the-mitingu-big-conference-invited (event-name-invited), but don't send it to anyone who has already registered, so the tag for that would be the-mitingu-big-conference (event-name).
Once you're done, then the email will be scheduled to send to everyone you've already invited, except those that have already registered.