In one study, two thirds of trans-identifying young people had one or more friends who were also trans.
Lisa Littman’s 2018 study  found that 66.8% of adolescents and young adults who identified as trans belonged to a friend group where at least one other person became gender dysphoric and came out as transgender.
 Littman, L. (2018). Rapid-onset gender dysphoria in adolescents and young adults: A study of parental reports. PLOS ONE, 13 (8). [Link]