Slightly anecdotal, but I used to be a gang intelligence analyst for law enforcement. In the Commonwealth of Virginia, gangs are defined as having: 1) a shared identifiable name or shared identifying sign or symbol, 2) members who individually or collectively have engaged in the commission of[..] two or more predicate criminal acts, at least one of which is an act of violence, and 3) have committed criminal acts which are in furtherance of the gang. The predicate acts are defined separately as a bunch of things, some logical and some less so.

The Boy Scouts of America, for example, meet criteria 1, but don't meet criteria 2 or 3 (I hope), so are therefore not a gang.

Alternatively, a person who associates with a known criminal gang and commits a violent robbery in order to obtain food for their kids, would not have their actions treated as gang-related crime because the act was not made in furtherance of the gang, but rather in furtherance of their person. If the same act was committed as a gang initiation, than it could be considered as gang-related crime.

A traditional Frat does not meet the criteria, in my mind, because there is insufficient evidence that they are committing violent acts in furtherance of the organization rather than for their own personal gain. You would have to argue that assaulting women is specifically perpetuated as part of that particular Frat's culture (not just Frats in general), and that members knowingly commit sexual assault in order share in or improve upon that Frat's culture and it's success.

I would also argue that a sub-group of wrong-doers who are part of a larger group of non-wrong-doers does not inherently define a gang. A group of bad cops doing bad things isn't inherently a gang, nor does it make cops a gang. A better example would be the CRASH cops from Rampart who essentially created distinct group within the police department, with names and symbols, who committed a variety of violent and non-violent crime in furtherance of the group itself, not the individuals or the police in general.