Ms. Cromer has supported Mr. Chalamet, 30, since 2018, when she was captivated by his performance as Elio in Luca Guadagnino’s “Call Me by Your Name,” according to a profile of Ms. Cromer in The Wall Street Journal. She started the fan group on X that same year, and has since served as a de facto publicist for Mr. Chalamet, highlighting his work in lengthy posts on social media and analyzing his career choices in essays on Substack.