As it sits now, if you want to share a photo to Google+ from your iPhone, you need to take a photo with the Camera app, leave the app, and launch Google+. There really isn't any easy way to communicate with Google apps unless it's a third party developer that's went out of their way to include support. According to the Google+ Developers Blog, Google plans on changing that with a Google+ iOS SDK for developers.
Now your users can share with their Google+ circles, directly from your iOS app! In addition, the native in-app share box supports image and video attachments.
While this won't affect native iOS apps such as the Camera app, it does give iOS developers great resources for implementing Google+ into their apps. If third party camera apps want to implement direct access to Google+, they now have an easier option to do so with the Google+ iOS SDK. It will also include token support so developers can verify the identity of the devices using their servers.
Hit the link to read more and to download the Google+ iOS SDK.
Source: Google+ Developers Blog