Settling on a good party playlist can be difficult. Say you've got friends with different tastes in music, and your music library doesn't have enough danceable tracks on its own that everyone can enjoy. If it's a big party, you don't want just anyone touching your iPod, and you also don't want someone with questionable taste to hijack the music for the party.
With the GrouPlay app, anyone with access to your Wi-Fi network can contribute songs from their own music library to your temporary party playlist, provided they have an iPhone 3GS, 3rd generation iPod touch, iPad, or later. Once the songs are uploaded, the real fun begins...