So, Athiests, Do You Say Grace?

Silently sit there, and politely wait for them to finish before starting to eat. In my experience with religious people, they don't call one out on not praying, but it would be deemed rather rude in my mind to do anything other than be silent if the person who invited me to eat with them is praying.

I tend to take a "don't be a jerk" philosophy towards people, and thus people I befriend (religious or not) tend to be the same.