I think you are well within your rights to ask people who wish to see the baby to have the booster if that is how you feel. If they don't want to then that is certainly their right, and they can wait until the baby is a bit older before meeting them.
It may offend/upset some people, but i can totally understand your concern if a friend lost their baby to WC. If you explain the situation to them they are more likely to understand your motivation to ask.