Just pick up an Apple Airport Extreme WiFi base station. They're "mesh"-able so you can stack additional units into a network to expand it out -- but for your use case, you can set up a guest access network that you can change the password to. This way, your base network does not have to change their settings at all, and you can simply set up the guest network on a per event basis with new credentials.
Support article on guest network: http://support.apple.com/kb/HT3477
Upside, you also get some wicked strong WiFi, dual band 802.11n/a/b/g.
Downside, max out @ 50 clients (according to product page)