Other forms: fraternizations
The act of hanging out or being friendly with other people, especially people you don't usually associate with, is fraternization. Your field hockey coach may discourage fraternization with the opposing team.
Some fraternization is innocent, friendly fun, like the fraternization you do after work with your co-workers, people you wouldn't necessarily socialize with if you didn't happen to work together. There's also "fraternization with the enemy," which has a more sinister, treasonous implication, particularly in the military or government. The word fraternization comes from the Latin word for "brotherly," fraternus.