See here:
assets2.hrc.org/files/assets/resources/Adams-Amicus-Brief.pdf?_ga=2.120105480.1087494444.1551613893-770574290.1551613893
Basically tech companies including Apple, Microsoft, Twitter, Tumblr, Yelp, Spotify, Patreon, Ebay, Lyft, Airbnb, IBM, and others, as well as some other woke signallers such as Deutsche Bank, GSK and others.
The brief:
"This amicus brief is submitted on behalf of some of the largest and most wellknown companies in the United States to address the rights of transgender students under Title IX of the Education Amendments of 1972 and the Equal Protection Clause. A complete listing of Amici has been provided to the Court. Amici share core values of equality, respect, and dignity for all people, regardless of their gender identity. Amici support and defend public policies that protect civil rights and foster acceptance and equal treatment for all of their employees, their customers, and the families of both. Amici recognize that diversity and inclusion are good for business.
Discrimination in any form against transgender, gender non-binary, and intersex people imposes enormous productivity costs on Amici and their employees and undermines their ability to attract, retain, and remain competitive with the best talent"
The case relates to a biological female who identifies as a boy and is only allowed to use gender-neutral or female bathrooms. It is claimed that there is a shortage of gender-neutral facilities meaning delays as a result.
I would note that Title IX was passed in 1972 to protect against sex discrimination and I am not aware of any precedent extending it to 'gender', though Obama did attempt to do so by Executive Order, this has been revoked by Trump.
This brief is by the HRC, which I'm surprised to learn is actually an LGBT charity as it seems to mostly address T stuff.