Google's YouTube party app is available without an invitation:
You no longer need an invitation to see whether or not Uptime, the YouTube social viewing app, lives up to the hype. Google incubator Area 120 has made the app accessible to anyone in the US with an iOS device -- sorry, there's still no Android version. The app has gone through a few upgrades since its debut, including a new home screen to help you discover videos, support for music videos and an option for finding friends through Facebook. Either way, the core concept remains the same: you can either watch videos 'live' with your friends or watch their reactions to a previous session.
While this represents a formal launch of sorts for Uptime, it's safe to say that it remains an experiment. Google doesn't publish Area 120 apps under its own name or otherwise put them on the same level as its official apps. With that said, we wouldn't be shocked if some of Uptime's shared viewing features eventually find their way into the official YouTube app.
Source: App Store