What a load of miseries people on this thread are.
I'm amazed that people can't see friends widening an invitation to include others as a positive. Isn't there a good chance that the friends of your friends are likely to be people you will like too?
Unless it had been explicitly said that this was to be a small, intimate meal, I think it was fine to invite others.
To call these people 'crashes' just sounds wrong. I'm sure that if they had any sense that you guys would feel so closed to them being there, they wouldn't come. Perhaps they asked your friends if they were free for a meal this week, and your friends said they were having a meal with you, so they would be welcome to join you.......not exactly crashing.
It was a shame that you heard about it via social media......but that is what happens with social media....you sometimes near things before the person has had a chance to speak to you personally. I'm sure they intended to let you know some others would be joining you.....whether they should have actually asked permission first, seems to be the thing that is upsetting some people. Isn't it possible to just be a bit more flexible?
Anyway, why don't you go with an open mind. You might find you have a great time, it takes your mind off your bad week and you at the least enjoy their company and at best find you make some new friends.
Hope you are able to go with an open mind and have a great evening.