If the UVa frat sues Rolling Stone, "they are opening up every young man in that fraternity to scrutiny � their drinking habits, and I�m sure some of them are underage..."

"... their sexual habits, and their overall conduct....  It just seems like there�s a whole host of issues that could be there, and it would be unfair and unwise to subject these young men to that," Charles Tobin � who specializes in defamation law � told WaPo's Terrence McCoy.

In addition to that skeletons-in-the-closet problem, McCoy points out the problem of a group claiming defamation:

The Rolling Stone article doesn�t specifically name any student beyond pseudonyms and descriptions that aren�t matched by any member of the frat house....

For any group to have a justifiable claim, wrote Ellyn Tracy Marcus in the California Law Review in 1983, the group needs to be small. "As group size increases, courts become skeptical that the defamation could reasonably be understood to refer to any individual group member. � Reasonable persons do not take literally statements defaming groups of people, and understand such statements only as generalizations or exaggerations."

I'm looking at that law review article, and it's talking about, for example, a case where individual D.C. taxi drivers tried to sue The Washington Post for an article portraying D.C. cab drivers as rude louts. Rolling Stone besmirched the name of a specific frat. Anyway, McCoy also links to Eugene Volokh's analysis (from last December, before the recent news that the police investigation has found absolutely no evidence to support the rape anecdote told by Rolling Stone). Volokh discusses "defamation of a group," but then moves on to the separate topic of "Defamation of the fraternity": "Corporations and unincorporated associations that have recognized legal identities (such as unions, partnerships and the like) can also sue for defamation that causes injury to their organizational reputation, independently of whether any member was defamed."

I'm not a libel law expert, but I see the "group libel" problem as addressing whether individual frat members could successfully claim to have been defamed because their frat was defamed. If the frat sues as an entity, there's no "group libel" problem.