The fratmosphere is the aura that permeates the fraternity/sorority house, party, and social event. Its use is not limited to just describing fraternity related functions and fraternity brothers are not uniquely talented in creating a fratmosphere.
An event where there is a fratmosphere is often characterized by men and women dressing very much alike (possibly popped collars on the men), the scent of the cheapest beers (e.g. PBR or The Beast), "hunch punch", and women posing in the "sorority squat" for photos.
"Dude, I went to this party last night with some of my college buddies and we hit the two-story beer bong just like old times... I'm not usually one to get that wild anymore, but I got sucked into the fratmosphere of the party.