The sexually suggestive email, to freshmen women at the University of Pennsylvania, created a firestorm:

"We're looking for the fun ones," read the poem from, inviting recipients to a "Wild Wednesday" party in an off-campus house shortly after classes began last month.

"Tonight is your first showing. So please wear something tight."

The invitation, alleged to have come from an off-campus fraternity known as "Oz," drew ire from female students who labeled it "rape culture." More than 900 female members of Penn's sororities and co-ed fraternities signed a letter calling the email "offensive" and "sexist."

Penn's Interfraternity Council, the student governing body that oversees the 28 fraternity chapters on campus, added its condemnation, but pointed out that Oz is not part of Penn's official fraternity system.

"It's outside my job description to deal with them at all," said Wharton senior David Moore, 22, council president.

The case raises a thorny issue for Penn - and other colleges around the nation that have struggled with so-called "underground" fraternities and sororities: How to deal with a loosely associated group that isn't recognized by the school and whose members can be hard to confirm, especially when they behave badly.

"These organizations really don't have any connection to the institution other than the fact that their members are Penn students," said Hikaru Kozuma, Penn's associate vice provost for student affairs.

Many underground groups are remnants of official fraternities or sororities that were kicked off campus for violating hazing, alcohol or other policies, which makes their existence worrisome, according to the North-American Interfraternity Conference, based in Indiana.

"Chapters that have lost national recognition and continue to operate are very concerning as they have been shown to pose health and safety concerns for students," said Heather Matthews Kirk, spokeswoman.

Some colleges have sought to ban them. Amherst in 2014 said it would prohibit students from being members, following an earlier complaint from a student that underground frats created a sexually hostile environment in violation of federal law.

"We do not believe that underground fraternities exist anymore," spokeswoman Caroline Hanna said last week. "No students have violated our policy, so we have not had to discipline anyone."

Lafayette College in Easton also announced a crackdown in 2012 after a student died from drinking allegedly in connection with an underground frat, though no one was ever charged. The college declined to make anyone available to discuss the issue, but pointed to its student handbook that prohibits students from any on-campus activity, such as recruiting or pledging, with an unrecognized Greek organization. The prohibition extends to the use of college email, phone or other technology.

Kozuma questioned how such a prohibition could be monitored or enforced.

Penn focuses on the individual rather than groups or associations, he said.

"Your behavior is your behavior regardless of your affiliation," he said.

Penn continues to look into the email allegedly sent by Oz, he said. He declined to discuss specifics about the incident.

If Oz were an official frat, Penn could have brought members in and talked to them and they may have faced sanction by their national chapter, Kozuma said. If incidents had continued, Penn could have taken more action, he said. In the case of underground groups, Penn can only discipline an individual or individuals if they can confirm a violation of school policy.

"It's kind of frustrating for us," said Amanda Silberling, 20, an English major from Boca Raton, Fla., who led the protest against the email.

It's frustrating for the interfraternity council, too.

"The unfortunate reality of the situation is that no one holds off-campus groups accountable for their actions," council president Moore wrote in an op-ed piece for The Daily Pennsylvanian, the student newspaper. "There is no way to reprimand them officially because they're unofficial."

Penn's Office of Fraternity and Sorority Life on its web site warns students and parents about these "underground" associations and advises against membership. The groups engage in "high-risk activities" and do not have regulations against hazing or the use of alcohol, Penn states. It names seven groups: Oz, Apes, The Owl Society, Theos, Tabard Society, Phi and Oax.

"Their members live in, and host parties at, nuisance houses off campus and are known to cause problems in the community," Penn warns.

Villanova University carries a similar warning on its web site. The school also states that underground groups operate without personal liability insurance or affiliation with a national organization and do not have tax-exempt status, putting their members and their party guests at risk.

Jonathan Gust, a Villanova spokesman, said the university isn't aware of any underground frats currently operating.

"They tend to phase out over time as students graduate and interest fades," Gust said.

For some students, the off-campus groups offer a less restrictive and comfortable association.

Sophomore Abby McGuckin chose the off-campus sorority OAX because of its less formal alternative to the strict sorority rush process and because she got to know and like its members. OAX was formed by some  members of Alpha Chi Omega (AXO) after Penn sanctioned it for violations of the drug and alcohol policy almost two years ago and members went off campus.

McGuckin said she's never seen hazing or forced drinking.

"I consider myself a strong-willed person and if I had found any faults within my system, I wouldn't have joined," said McGuckin, 19, an urban studies major from Radnor.

Some of the groups at Penn have been around for decades and, like OAX, are outgrowths of official frats or sororities that ran afoul of university policies. The APES were formerly Alpha Epsilon Pi, booted from campus in 2012 for a violation of Penn's anti-hazing policy, according to the Daily Pennsylvanian.

Oz is believed to be remnants of an old Zeta Beta Tau. It's hard to get anyone from the group to respond to the allegations.

An email to the address drew no response. Neither did a note left in the door of the house in the 200 block of S. 42nd Street where women were invited to the party.

"I think they think if they don't put their name out there, they can't get in any real trouble," Silberling said.

But Silberling and a group of her friends made sure whoever sent the email was held accountable, at least in the court of public opinion.

When she received a screen shot of the invitation from a freshman, she shared it with her Facebook chat group.

"We thought the organization should be publicly called out for that and for targeting 18-year-old girls, given that they were sending this email to freshmen," said senior Hannah Judd, 21, a music major from Brunswick, Maine.

The party invitation promised beer and wine. Judd fumed.

"They are saying in so many words we are going to get you intoxicated in order to do what we want with you," she said.

That night, about a dozen women met and printed hundreds of flyers with a copy of the invitation and stamped it "rape culture." They posted them in college houses, on bulletin boards, on the Love statue, along Locust Walk.

As their protest got media attention, Gov. Wolf and Sen. Bob Casey tweeted support. Actor Ashton Kutcher called the email "disgusting" on Facebook.

Penn administrators offered praise, too.

"Nothing, nothing, nothing is as powerful, poignant, and preventative as eloquent peer speech against the speech of other Penn peers whose speech they find abhorrent, reprehensible, and vile," said Valarie Swain-Cade McCoullum, vice provost for university life.

Off-campus groups also added support. McGuckin, of OAX, helped to organize the opposition letter signed by sorority members. Moore, the interfraternity president, said an off-campus frat has asked Men Against Rape and Sexual Assault to speak to its members. The council also is renewing efforts to insure on-campus frats are educated. He called on all students to take a stand.

Silberling and her friends are ready.

"I'm really glad," she said, "that at least we started this kind of dialogue."

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