Chapter membership requires membership in IAC.
IAC is part of EAA, so the "Join IAC" link launches the EAA membership page that also enables joining IAC.
EAA and IAC membership is required to compete in contests and serve as a judge.
Attendance in a Chapter 67 aerobatics camp (see Coaching and Camp) requires (EAA/IAC and) Chapter 67 membership. Chapter 67 members can attend the Chapter 67 judges school for free; some other chapters charge to attend their judges school.
After joining IAC, check out the Join or Renew Chapter 67 page