(First a disclaimer, the developer of this app is a friend of mine. That doesn't mean that I don't think it's a cool app though.)
My friends at Twocanoes have released a little app for iOS called Guest Pass. It's pretty simple but works well. What it does is generate a pass for your friends on iOS or OS X to use to access your wifi network at your house.
To use it you fill in some blanks and it then sends them a configuration profile that sets everything up. You can set the pass to expire too and it will auto-delete when it does.
I beta tested it and used it to get my friends that visited for Thanksgiving connected. Now that I've sent them one pass I can re-issue it next time they visit. You could use this to issues passes for bigger things like a conference too and the passes would self remove when the conference was over.
Here's some links to his webpage and to a demo video. (Not going to include App Store links here as that would make this feel more spammy than it does already. I'm just trying to help get the word out and am not affiliated with Twocanoes other than friendship!)