From trying this out in the test group, it seems to me that you could mention this in the Description field...just get into the habit of appending a sentence saying:? "Please click the 'Are You Going' link above to RSVP."
Unfortunately, if you permit subscribers to create calendar entries, this option becomes more problematical. It's also not particularly good for subscribers who might be getting plain-text message delivery, but I see no way around that.
?
Hope this helps,
Bruce
Bruce