huh? What's the matter with you people? It's nothing to do with plural or belonging. "I'm a Jehovah's witness" is simply bad grammar. If I was a witness at a trial for my friend Brian, I wouldn't say "I'm a Brian's witness" - I'm Brian's witness or one of Brian's witnesses. The 's stands for his.
When I was in, I used correct people whenever they said "I'm a Jehovah's witness" (I'm a pedantic bugger about that sort of thing), not that they ever took any notice.
Hello?