Like Jim said, if the wi-fi network doesn't have enough signal strength it won't be displayed. Are the other people using iOS devices? If iOS devices are seeing and connecting to the networks and you are not, the troubleshooting basics to follow are restart/reset/restore, in addition you'd want to reset the network settings. If those things don't work, then a trip to the Genius bar would be appropriate.
http://support.apple.com/kb/HT1430
http://support.apple.com/kb/HT1414
You can also look through the applicable parts if this kb
http://support.apple.com/kb/TS1398
Hope this helps.