When one is walking by them selves and lonely, To prefriend is to walk close enough to a group of people so that passers by assume you are with the group, but not so close as to make the group think you are following them.
Note: If one actually believes that they are with the group, they are not "Prefriending", they are just Delusional.
To "Prefriend" to be with someone,
One can be "Prefriending" to be with someone.
Person 1: "Hey look at Pete, I didn't know he was friends with the Jocks"
Person 2: "What?...Nah he's not, He's just "Prefriending" to be with them"
Person 1: "Pfft, What a loser"
Ultra-popular Kim: Heather, why did you friend Jill last week, she's such a loser?
Just-as-popular Heather: Because she worships me just like my other 1200 friends on Facebook, confirming my popularity and ensuring I'll win the vote for Homecoming Queen.
Heather grows up to become the skanky trophy wife of a used-car salesman. Jill grows up to be a lawyer and makes partner at a major law firm.
Clyde - "You should totally pre-friend her so you have something to talk about!"