Some of the group’s members decided to form an offshoot group in which students could share obscene, “R-rated” memes, a student told the Crimson. The founders of the messaging group demanded that students post provocative memes in the main group chat to gain admittance to the smaller group.

But from wording later, the examples given seem to be from the private group? I'm also unclear if the private group some some offshoot of the main group within Facebook (that is, whether it's some Facebook sub-group feature, obviously they are members of the main group).

In any case, Harvard says they reserve the right to reject admission based on "honesty, maturity, or moral character" so if this information is brought to their attention, then it falls under what they consider valid acceptance criteria. It's no different than if they Google searched the applicant initially and found verifiable information that put the student in the same light.