right-click the wi-fi signal icon in the status bar and select ‘create network’
This is useful if you are trying to run a project which requires WiFi – like touchOSC – and you are on a restricted network – at a school for example. You make your own network which is separate from the Internet.
at dummies.com
http://www.dummies.com/how-to/content/how-to-create-a-wireless-ad-hoc-network-between-macs.html