It would be really helpful for event planning if registration could be limited to contacts that received an invitation. I have run into instances where we have a limited number of tickets available for an event, and a contact has forwarded the invitation to coworkers. The coworkers RSVPd and registered, taking up the tickets we had intended for other contacts. Considering you have to select a list for sending out the invitation in EventSpot, having an option to restrict RSVPs to contacts on that list would be helpful and prevent events from reaching capacity before all invitees are able to RSVP.
We can't prevent contacts from forwarding the invite, but preventing uninvited guests from registering would be helpfu;/