To meet with a group of people; hangout.
Comes from carpooling where you get in groups of five to save gas.
We have to drive pretty far, so let's five-up at my place 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.
To spend an entire night together and not miss a single high-five.
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."
There was a minute there when I thought Brad was going to miss my hand, but he made it, so we were five-up.