(some cases of this: Argent Crusade has undead members, as does Darkmoon Faire. I doubt no one really has bothered to recruit Augustus the Touched either. Meryl Felstorm has never been a part of the Forsaken either.)
edit: my point is that since not all undead are Forsaken aligned I doubt undead would just kill someone for refusing to work with them initially. After all they could be converted later. Every undead IS quite probably risen to be a recruit, though.