No message if sign up is disabled for a given event

In rsvp_signup, a user is alerted if they response to an invitation but signup is disabled for the given event. I don't believe it makes sense to notify a user of this, since the integration between signup and RSVP is a backend process. The only thing the user knows if that they are responding YES to an invitation. As long as that is permitted, no message should be sent.

*I'm considering suggesting that the user be signed up for the event regardless of the status of signup for the given event. That way, if the event admin later turns on signup, it will be synced with the rsvp list of people who have accepted.*