Response card etiquette?

A friend recently had a wedding that had a limited guest list. They both have big families and are young with not a lot of $ and the location had limited seating. The RSVP cards said:

“We look forward to celebrating with you.
The favor of a reply is requested by April… [etc]
M_________________
Number of persons attending ___”

In my opinion, this is standard. However, some of her college friends (who would have plenty of friends and people to talk to at the wedding no matter what) RSVP’d for uninvited guests (not husbands/wives/partners — just friends) and one of them RSVP’d for 1 and still thought that meant she was allowed to bring a guest just for the fun of it. If it were just a matter of people RSVP’ing for more than is ok, I’d just call them, but I really can’t afford to have out of the blue guests at my wedding.

Is there a way to word these cards so that this can’t happen? I’ve heard that some people write the line:

“We have reserved [#] seat(s) for you.”

yea or nay?
For clarity: I *do* know the procedure for what people are *supposed* to do as far as RSVP’s. But since it is clear that some people don’t get this, I’m trying to find ways to make it abundantly clear even to those who have never attended a wedding obefore.