The following if copied from about.com:
Some guests will assume that their guests/dates/friends are of course invited, regardless of to whom you address the card. You can avoid these assumptions by writing “___ of ___ guest(s) will attend”, and then pre-filling in the second blank with the number of people that you are inviting. Some may find this slightly distasteful, but it certainly does get the point across!
Has anyone done this? Would anyone do this? What other ways did you word RSVP cards/numbers when inviting multiples within a family? Thank you.