The phrase "fraternity brothers" refers to the members of a male social organization called a fraternity. They are like a close-knit group of friends, who often live together and participate in social and academic activities as a group.
Summer: Calder spends the summer working on wood and wire sculptures at the Peekskill, New York, farm of J. L. Murphy, the uncle of fraternity brother Bill Drew. (calder.org)
The two met during their days as Kappa Sigma fraternity brothers at Stanford. (businessinsider.com)
He says he understands it can apply to former fraternity brothers who work in tech, like the ones who invented Snapchat, and people who work in sales technology. (inc.com)