What?

Something interesting happened today at luch (which was about thirty minutes ago from when I started typing this). These two guys walked up to my table during lunch. One said "Hey, all of you know who I am (We didn't know who he was). I'm having a party tonight at my house at seven. There will be chicken, food, pizza, and all sorts of stuff. Its invitation only so take this (*he hands out slips of paper*) and I'll see you guys tonight." And then they walk away.

Nothing was on the paper. It was blank so we were thinking 'What?' except Joe. He thought it was written in invisible ink and you needed a black light to see it. Poor Joe. Nothing is on the slip of paper.