Glassboard is a Cross-Platform Private Chat Board for iOS, Android and WP7

iOS/Android/WP7: If you're working on a project with a group of people and need a means to share new data, photos, ideas, or relevant links but you don't want to start a mass email thread, Glassboard is a free mobile app that works like a private social network.

Right away you'll be asked to set up your first board, which is a stream of content only people you invite can view. You can set up as many boards as you like and invite people to specific projects. It comes with a few nice settings, including push notification "quiet hours" that will automatically disable notification during times of your choosing. It's also non-existent on the web and everything is stored on Glassboard's servers, so if you're working on a project with a few people and want to communicate on your mobile but still have privacy, this should do the trick.