How to create an ad hoc Wi-Fi network in OS X?

I came across a problem today when there was a speaker in my class. The speaker was from the industry and had a company provided laptop which was not able to get connected to University network (some other problem which I had to raise in another question). As a solution to the problem, I thought I would create the public Wi-Fi, using my MacBook Pro, without a password so that he can get connected directly. However I am a little embarrassed in not finding a option do so. Can anyone give me some solution if one exists.