Note: this event has the rsvp feature enabled. Click here to view and manage your RSVPs.

From Rich
add a simple line of text that would appear under the edit tab, if an rsvp exists:
"Note: this event has the rsvp feature enabled. Click here to view and manage your RSVPs."

or alternatively:
"Note: you have not set up the rsvp feature for this event. Click here to set it up."