Unlike Facebook and Twitter, Google+ has the fortuitous distinction of having been optimized for mobile from its inception. This means that the Google+ app is not only a faithful, thumb-friendly port of the web version, but actually has its own set of cool features.
"Huddles" a core component of Google+ for mobile, is like a group messaging service on steroids. You can add any contact or an entire circle (up to 100 can participate) to a huddle and then message them all at once. If people join a huddle, then their messages will be seen by everyone else in the huddle, too.
There's also "Instant Upload," which is a simple but ingenious feature that works with your camera phone. With the Google+ app, any photos or videos you capture are automatically uploaded and stored on the cloud. The service wisely doesn't share the images with anyone, but you get them in your own private album and from there can send them off to your friends and circles as you please.