We're all in...all five of us out there together, in the wild, getting some strange, like a sex fist! 1, 2, 3, 4, 5 of us together, make a fist: Schmidt, Nick, Coach, Winston, Jess, sex fist when you put them all together! Tearing our way through the reception! It's a metaphor for togetherness.
Jess said this when the group decided to make a pact that they would all get laid after a friend's wedding. In her typical fashion, she didn't realize the context of this statement until after she had rambled on a little.