Don't give up on it so easy man. Give it some time, especially closer to your birthday when i think it will gather more interest.
though an xbox live party is a bit unorthodox.
5 people have already said they are going to show (how many actually I'm not sure).
Just have some patience, just because people haven't posted doesn't mean that not many people are interested.
Its difficult to get people to come on to xbox live, it isn't something you log onto often unless you are really into it.
But for the sake of promotion ....CMON PEOPLE lets get some people to join this thing for some xbox live party fun
lets turn this into a little bit of an ocr social gathering (minus the cross country trips )