Unwanted Guests

We are throwing a 50th anniversary party for my in-laws and are concerned about uninvited guests. I know etiquette says to only include the invited people on the invitation and others should not come, however, we know for certain that if we don`t specify that only those invited may come, they will invite additional people themselves. Any suggestions on how to make it clear on the invitation that additional guests are not welcome? I was thinking to word it nicely on the RSVP card.