RSVPs and "Best Persons"

Margaret says…
 
 No sane person holds a large, and staggeringly expensive, gathering without a headcount. Therefore, it’s right to assume that your hosts are waiting to hear from you.
 
 They didn’t include an RSVP prompter because, at one time, it was considered impolite. Printing an RSVP notation or card implied that guests were unaware of the proper way to respond.
 
 Unfortunately, this nicety confounds guests who have come to expect not only an RSVP notation, but also a card, a pre-addressed, stamped envelope, and a “drop dead” date by which they should respond. Even then, some guests can’t bring themselves to mark a little X in the correct box and drop it in the mail. This forces the hosts to call and snap, “Are you coming or what?” when the caterer demands to know how much chicken to order. It’s reason enough to elope.
 
 As you surmised, your hosts are expecting a handwritten note indicating whether you’ll be able to attend. This is the traditional way to respond, regardless of whether the invitation is handwritten, engraved, or printed on ducky-bride-and-groom novelty stationery. To accept, center the note on your personal stationery, and write:

 Mr. and Mrs. Friends
 accept with pleasure
 the kind invitation of
 Mr. and Mrs. Wedding
 for
 Saturday, the eleventh of October
 at five o’clock in the evening.

If you can’t attend, the note should read:

Mr. and Mrs. Friends
 regret that they are unable to accept
 the very kind invitation of
 Mr. and Mrs. Wedding
 for
 Saturday, the eleventh of October.

For cocktail parties, you can call.
 

* * *

Margaret says…
 
 No matter where honor attendants are standing, the titles remain the same. Therefore your fiancé’s sister is his maid or matron of honor, and a gentleman acting as your honor attendant would be your best man.
 
 The gender-neutral term for bridesmaids or groomsmen is “attendants.”