1. Make sure you give the users your internet IP address and not a local IP address! See the 'What's my IP address?'.
2. Does your firewall block incoming traffic for this application? Most of the time you'll have to give the server full access to the internet.
3. If you have a router in your network, forward port 80 to the IP address where the server runs on.
For more information how to configure you type of router please check out this website: http://www.portforward.com/routers.htm
This site is dedicated to port fowarding and has detailed descriptions for almost every type of router.