Do unattached objects contribute to a planet's gravity? If you could measure Earth's gravity with infinite precision, and did so both with Earth and its full population, and Earth with no people, would there be a direct difference in gravity? Is this even a valid way to think of gravity?
Yes, it would change the attractive force by 10^-14 if there was a world without people. 

Depending on the distance, you can measure gravity of people and buildings, in fact they used gravity to measure gaps in the pyramids of Giza once. 

Today gravity surveys can detect caves and reservoirs of 2m to 50m at 2m to 150m depth, so in the future, we will be able to discover many caves and fossils with nanotechnology gravimeters.
 http://physicsworld.com/cws/article/news/2016/mar/30/new-gravimeter-on-a-chip-is-tiny-yet-extremely-sensitive