Freshmongers are usually female, most often obese and hated by their own classmates. Freshman look up to them because they are upperclassmen and think they are cool for hanging out with one.
It is very likely that the Freshmonger won't have a date to the prom, so they invite one of their Freshman buddies so they won't be so lonely.
Freshman Girl 2: We r soo hot! Let's go be sluts!
Freshmonger: Do you guys want to go to prom with me? Nobody wants to be my date!! Isn't that weird? I'm so hot!