Sit silently, do not bow your head ( a sign of submission and concurrence),
do not close your eyes ( another sign of agreement with the prayer). Some families hold hands at this time. I will join in that. Some all say Amen at the end. I do not say Amen.
Next Question: What do you, as the host, do when you have a family to your house for dinner who always says a Christian grace before starting the meal?
a. Since this is your house, as a sign of respect, they should be fine with skipping it. Just start passing the bowls.
b. Allow them to say their grace/prayer at your invitation but not join in yourself.
c. provide your own alternative grace/prayer. Such as:
"Let's all join hands. ( optional) Don't bow or close your eyes. Look around and see the faces of friends and family and let us be happy to be together for this meal. Let's be thankful we have an abundance of food due to our economy, our hard work, and science for creating fertilizers, seeds, machinery, and transportation that brings it to our table. Bon appetite! " or something less corny !