The obvious way to keep it private is through use of firewalls and not publishing the tracker outside of your office. Trackers themselves do need to be published, as I remember.
The basic problem that BitTorrent is solving is how to get thousands of people to efficiently download at high speed from a point source on the Internet. The bottleneck in this situation is the connection from the server to the Internet; after that, the packets go in all different directions on different connections. In an office LAN, the equivalent connection is going to be the Ethernet cable from the server to the switch, and the easiest way to reduce that as a bottleneck is to use gigabit Ethernet, not to use a complicated software solution. It's also possible that the bottleneck is the switch itself that is routing the packets; in a case like this, the speed of distribution will entirely depend on the switch, and BitTorrent will actually slow things down (on a local and global basis) by adding overhead to the file transfers.