What are the regulations on flying UAVs over people on the ground?
The general rule of thumb is "don't do anything hazardous for people or property on the ground". 

In the US, do not intentionally fly over unprotected persons or moving vehicles, and remain at least 25 feet away from individuals and vulnerable property.

Based on whether your project is for recreation or not, how heavy the drone is,... you may not be able to do the same things. The FAA website has a page titled ``What Can You Do With Your Unmanned Aircraft?'' that gives details on what you can and can't do based on these parameters.

A document published by EASA states that for low-risk drone operations:


  Flights above crowds are prohibited, but flights above people not related to the operation in cities or populated areas is allowed.


Refer also to local laws. A number of states and municipalities have passed or are considering limitations on unmanned aircraft.