A well-known national transgender policy group finds itself having to rebuild after a mass exodus of staff members amid discontent over its leadership, claims of retaliation for labor organizing and concerns about minority hiring and outreach. The group, the National Center for Transgender Equality, has lost about two-thirds of its staff in just weeks, Mara Keisling, the group’s executive director, said on Friday night. At least eight staff members resigned as a result of frustrations with the organization and a failed attempt to oust the executive and deputy executive directors. Those former staff members explained their reasons for leaving in an open letter published Friday by Out magazine. Tensions at the center have been building for years, according to the letter.