I am having a rather informal wedding, the "formal" is at the church but the reception is in a barn with yard games and what not, so we aren't taking the reception too seriously. So for our RSVP's, I did the usual M--- thing to respond, but then I added a section saying "--- of --- attending" where they would fill in the first # and I would fill in the 2nd number. My family is not exactly one to follow ettiquette basically, but we have an age restriction on guests so I want people to understand that who is on the invite is who is invited, not and guest, not and family. But is that tacky? It's also just helpful in general to know that Bob and Sue can make it but their oldest son johnny can't, instead of just checking the "greatfully accepts" spot where you aren't 100% sure that means EVERYONE can come.