got my first plus one and the invites havent gone out yet

***...My best friend's sister IMs me the other day to tell me she's 'head over heals' with the 50th guy since I've known her (ok I'm exaggerating) and casually mentions that I can send the invite her 'plus one'..I was so caught off guard that I actually agreed to it.
...I really thought I would have just the right words to say when it happened to me...
But I was completely caught off guard on this one since I have'nt even sent out invites yet!!!