I think it is different between being a proctor and a marker. A proctor is only to administer the exam, and does not interpret, confirm nor deny the meaning of a question, while a marker grades exams and affects the pass/fail of candidates.
For markers, I guess they only allow full privileged members or fellows who have the charter and designation to mark exams as they will be marking exams at the CFA HQs and have access to confidential and material non-public information: exam papers.
Proctors may be members from the local CFA society who passed all exams (please correct if wrong). But, any person who qualifies to proctor in university exams could be a proctor for most professional exams even they are not related to the professional organization associated with the exam. For example, professors, graduate students who are assigned teaching assistant roles, senior admin of the university, or even senior bank officials who have signing authority, or other people with recognized professional status licensed by state/province e.g. accountants, lawyers, engineers, MDs, dentists, nurses, pharmacists, actuaries, etc.
For example, Society of Actuaries exams are mostly held worldwide at authorized locations of exam service providers such as Prometric, or at universities or insurance / consulting companies worldwide where exams are proctored by full members who are fellows of the society (FSA) or university professors and master’s / PhD students major in math/actuarial science, as long as the proctors have passed that particular level of exam they are proctoring.
If none of these available, then with other recognized professional organizations or exam authority of the host country. At the last resort, for countries do not have any secure and trustworthy locations, or at combat zones, then exams might be arranged to take place at the nearest U.S. Embassy or Consulate (i.e. on U.S. soil) if absolutely necessary.