Thanks for clarifying, Chris. I was overthinking it.
Just a few ideas:
Some groups have official histories, and a person in charge of maintaining archives.
Qualifications for membership might help - you can't join the American Bar Association without passing the bar exam. Being qualified wouldn't prove membership, but not being qualified would prove they couldn't have joined.
Names might be mentioned in news articles or legal actions.
If the organization was incorporated, officers' and directors' names appear on public records.
If you can't find proof, you still can mention the speculation in the biography.