Tell Your Friend thanks you, but you are sorry that you can't make it that day
Open the door, rudely snatch the invitation out of his/her friend and shut the door
Ignore her completely
Wait until they are done speaking before you start to talk
Throw a big long tantrum
Shout "I HAVE TO GO TO THE BATHROOM!" and create a big scene
Just get up and walk away
Politely excuse yourself and say "would you please excuse me, i'll be back in a moment"
Here's an interesting quiz for you.