ZB has good ideas.

Could you also say "please call before visiting as we may be resting". Then you can tell people you're too tired, but allow the ones you want to come. And if doorbell rings, don't answer (or put up a sign saying you're resting).

Or could the grandparents host an afternoon tea where you take the baby to introduce to everyone, but that way you don't have to do the entertaining and can leave when you need to?