I would point you to section 34 of IRPA in particular, which relates to inadmissibility concerning security. Sections 34, 35, 36, and 37 relate to security, criminality, war crimes, and organized crime.
That sets out the definition we all are guided by in terms of determining admissibility or inadmissibility to Canada. I can say that the courts' interpretation of the term “membership” has been fairly broad.