I asked dan if he wanted to come to my birthday party
Why did you ask dan to your birthday party?
I wanted to gather as many friends as possible
It is great that you want all your friends there to celebrate your birthday. They will appreciate being there for you and will have lots of fun.
I'm actually leaving the US for a few years, and I plan on telling everyone at the party. It'll probably ruin the mood but whatever.
I do not understand what you are doing.   You are only going to upset others and make life more difficult when you get back home.