Can anybody recommend a service/method to allow a one-click RSVP from a link in a mailer? (To be sent Campaign Monitor)

Looking for an event management service which makes RSVP quick and effortless for the users at the request of the client.

Eventbrite is a great service but the countdown timer and the need to register for even the free events is off-putting.

Can anybody suggest a quick way to setup an RSVP link which won't require registration/re-entry of details (by the recipients) that the client already has.

Any help appreciated!