It is actually fairly easy. I am currenty setting up an 'invite only' tournament myself.
If you want to have a stroke play tournament, just pair everyone up (2somes or 3 or 4) and have them invite each other. They record their own scores and report them to you.
If you want, you can use google docs to report the scores and record them. That might be overkill if they are all in your office.
Match play is more complicated, but see my blog for a simplified version.
good luck and good shooting