I read this story once but I can not find it any more.
It is about a skinny guy who invents this muscle growth potion and 1 girl who drinks who some how drinks it and transforms into this muscle bound chick. She invites her friend who also was weak and pushed around all her life too come over and lets her drink it too. They both then have the guy call his best friend who is a skinny nerdy girl who all was abused all her life come over. She came over to the guys house and goes down to the basement where she meets the 2 girls. She then was told to drink the potion but refused and spits it out after being forced to drink it. The two muscle girls then grabs her and shoves a tube down her throat and pours the serum down a funnel to her throat. As they are pouring the serum down her muscles start to grow and tear through her clothes and she pushes them away and and enjoys becomes a muscle goddess. She then goes to tease the guy.

Does anyone have any idea where I can find this story?

http://www.thevalkyrie.com/stories/johngrooms/family.txt

I think you're referring to that one.