Game Distributed Interactive Simulation (GDIS) was developed for the US Army as a system for training soldiers using virtual missions. The goal was to use low-cost commercial video game engines instead of larger more expensive simulation software. GDIS is not specific to any particular video game engine and has "adapters" for the Source, Gambryo, CryEngine, and Unity 3D engines.
GDIS also has an immersion component called MMIST/VIKENG. This incorporated full-body motion capture and voice and gesture recognition using commercially available hardware.
There is a huge explanation of the features and use cases for GDIS on the website at http://www.resrchnet.com/products/gdis/
The team of programmers was very small, and during the course of my employment with Research Network, Inc. our team fluctuated between 2 and 5 programmers. I became responsible for managing the team, doing demonstrations for clients, as well as writing code.
I could go into great detail about GDIS, MMIST, and SimBridge but I don't want to bore you :) So if you're curious, ask me.