Comes from the idea that people spending time together will have fun and give each other high fives. Since the geek culture boom in the 80's, it's become much easier for a high-five to miss. If one avoids this tragic accident of pop culture, one has been said to be "five-up."
Comes from carpooling where you get in groups of five to save gas.
Hey, you wanna five-up at the mall to go shopping?
Let's five-up at my place to watch a movie.