In case you were wondering:
If someone invites you to something, it is polite to let them know whether or not you are coming - even if you will be unable to attend. Then the host(ess) can stop bugging you. It's a win-win.
Additionally, if you can't hang out with someone when they invite you to do something, the appropriate response is: "I'm sorry I can't make it, why don't we hang out [insert alternate time]?"
Seriously folks, this is getting ridiculous.