Say you are planning a get together... a night out (bowling, movie, dinner party, clubbing) and one of the people you invited also invited someone else. Someone that you did not want to come along. What would you do? Would you deal with it, or would you un-invite that person, or would you just let that person know their guest was not welcome?
Now if you found out about the unwanted guest when the person arrived there is little you can do without being super cold and rude. And who wants to be cold and rude... really? I like Gossip Girl, but I could not actually be that much of a bitch to a person. But if you had advanced warning... there are so many options. It also depends on what you as a group plan on doing because certain social settings are easier to avoid people (clubbing, rollerskating).
I guess the worst possible situation is if you plan a date with someone and they bring a friend... clearly stating that it is not a date. I did that once... I was the person that did not get the hint that it was a date. I felt pretty stupid, when I finally caught on.