Just to add another perspective... I agree about putting your foot down with your MIL, inviting a group of people you don't know to your wedding is overstepping the line a little bit. However, with your friend, I can see how a misunderstanding could have occurred. In this day and age, not everyone is as familiar with invite etiquette and how specific wording indicates exactly who is invited. I have to admit that before I started planning my own wedding, I had no clue there were so many rules to it. I would suggest making an exception. Most brides are finding on here that many people who say that they are going to your wedding now, end up backing out when it comes time to actually book (dealing with that right now, myself), so you may find that you will still be under your count even with the additional guest. Also, it is no fun to travel alone and your friend probably just wanted someone to keep her company, knowing that you most likely will be busy with other things and having to spend time with multiple people throughout the trip. A few years ago, I was the "extra" guest to another wedding, and over the course of the vacation, got to know both the bride and groom quite well. To this day, they are some of my closest friends! I don't think that being gracious and allowing one extra person to join in will hurt your budget all that much, but it will hopefully create a better vacation for your friend and may even gain you another great friendship.