The game will be called Woorld and it appears to be a game where you can fill the physical space around you with digital toys and creatures to play with. It's kind of hard to tell what else to expect, because the developer, Funomena, describes the game like this:
The developer says that users will be able to collaborate with friends and family, but it doesn't sound like there will be an online sharing mechanism. Rather, you collaborate, by having everyone view your Woorld through a shared screen.
It seems like an interesting use of the augmented reality power of Project Tango. The game won't be fully released until the fall, but there will be a preview at Google I/O, so maybe we'll learn more.