How do I turn off all notifications at once and then turn them back on later (like if friend is playing game?)
If you just want to disable sounds, then you can flip the mute switch above the volume buttons. If you want to disable notifications that are based on an Internet connection (e.g. NYTimes, ESPN), you can put your iPhone in airplane mode which turns off wifi, 3G, and Bluetooth so you don't have an Internet connection. For apps that send notifications at a certain time (e.g. iCal reminders), there's no easy way to turn those off all at once other than turning them off individually in settings>notifications.