RE: Invitation Only Events
Hey Anne,
Here's how you can achieve this scenario (it requires a bit of customization).
If you're hosting the event website on the Dynamics 365 portal - then you need to:
1. Set the anonymous registration on the event to "No" (General tab on an event, Website section, set the "Allow anonymous registrations" to No) - this will make the event registration button route to the D365 portal login field.
2. On the D365 portals, set up invitations for your contacts (you can use this link to know more - docs.microsoft.com/.../invite-contacts ). This is a pure D365 portals capability.
3. On the D365 portals sign in page, remove the options for invitees to start a new registration (you'll have to customize the page). This way, the only option for them would be to log in or use the invitation code that you send them.
4. Now use dynamic emails to send the invitation emails out to your contacts, and include the unique invitation code on the email.
If you're hosting the event website on your own site - you'll have to tie in with the authentication provider that you're using for your own website. We document all of this in our Events developer guide here - docs.microsoft.com/.../self-hosted
Also, you can prevent the invitee from registering multiple attendees by disabling it from the event itself (General > Website > "Enable multi-attendee registration" to No)