At most dances, there's a big grindfest in the middle of the dance floor, where all the girls are back to back in a big circle grinding asses together, while all the guys make a bigger circle around them, jumping up and down and acting like frat guys.
Suzy: Did you go to the dance?
Suzy: I didn't see you.
Annie: I was in the middle..in the grindfest.
A dance where everyone on the floor (or off the floor) is grinding
Dude are you going to the dance? You should, because I heard its going to be a grindfest.