Are there any members of the International Astronomical Union that are active on the Astronomy Stack Exchange?
I am an IAU Member but a lot of professional astronomers, particularly junior scientists, aren't. The membership rules are slightly weird/archaic. You can only apply to join every three years when they are going to be holding the General Assembly (Past General Assemblies, next General Assembly will be in Busan, Korea in 2021). You have to submit a qualified application to the national astronomy body in the country you are practicing astronomy in, normally with a deadline of ~December in the previous year. (For me this meant to the National Academies as the USA National Committee for Astronomy (NCA) even though I am British and got my Astronomy PhD in the UK).