Steam is installed on a computer.
From thier you log in to your account, if another person say your son logs onto the same account on another computer it will log you off your account. Only 1 person (1 PC) may use it at one time. So if you wish to play together, or even both play single player at the same time completely seperate from each other you will need the two copies.
Its not so much CIV 5 limiting your ability to "cheat" as it were and only pay for 1 license, its just the way steam works. That only 1 person can use an account at one time, well thats the personal use package, If you run a lan-cafe or something I believe you can buy a much more expensive multi user package which supports many users per steam account. But I digress.
Your company shouldn't have any issue with Steam installed, if you don't want it loaded & online e.t.c while you are using it at work simply right click and exit it off the task bar. You can select it to not automatically load on start up too so when you want to use it for gaming only then load it up. So you know, it doesn't have to be on all the time in the background unless you want it to be. Updates will only download and install whilst the programme is running (in the background or otherwise) Which is the reason to leave it running, so you don't have to wait for any Updates to finish after turning it on when you should be gaming.
Edit : @ Gamemaster, tell your friends to get thier own copies. They should be at home on thier own computer with thier own copy playing with you over the internet. Lan based gaming is possible with Steam, but each person must have thier own steam account with thier own game, this is mobile however so they could still come over to your place and login to thier Steam on your spare computer and Lan with you. But to have only one copy of a game allowed to be played by multiple users over Lan on the same steam account, requires a special license. As now you are stepping into comerical cyber cafe territory, even if it's a personal LAN set up between friends, its the same as running your own cyber cafe free to your mates.