The story was broken in The Daily Beast, with author Asawin Suebsaeng noting that the source had asked that her name not appear because she fears retribution. Hence she's referred to by the pseudonym "Sarah" when she maintains (with some support from contemporaneous sources) that while attending college at George Washington University, she had at least two extraordinarily troubling encounters with Miller, including being slugged in the mouth and choked without consent during sex.
Unlike comic Louis C.K., who admitted to sexual misconduct during a 2002 comedy festival in Aspen, Miller and his wife, Kate, have co-written a response on Instagram in which they reject the claims in the Daily Beast story and suggest that the woman in question has a history of attempting to break up their relationship.

T.J. Miller and wife Kate Miller in a photo that accompanied his denial of the accusations against him on Facebook.
Kate Miller Instagram
Miller isn't the only Denver-related icon to be accused of sexual impropriety. In 2016, the story of former Broncos quarterback Peyton Manning allegedly pressing his naked butt against the face of trainer Jamie Naughright during his days at the University of Tennessee received renewed attention. Then, this past October, following Weinstein's shocking fall from grace, the report resurfaced when Naughright granted her first public interview on the subject to Inside Edition.
Manning has consistently insisted that he did nothing improper in regard to Naughright, and he's mostly avoided a PR hit for his alleged actions. It's too soon to know yet if Miller will do likewise. But the matter has gotten mighty ugly mighty fast.