Google's strong commitment to Circles, the contact grouping feature in Google+, is undeniable as the feature is being integrated deeply into Gmail.
Google executives have asserted repeatedly in the last few months that we would be seeing Google+ integration across the spectrum of its products. So far, we have seen just that from the tie-ins between Google+ and Reader to the social networking platform effectively replacing Picasa and Blogger.
These updates are really only noticeable and beneficial if you actively subscribe to Google Plus. And if you don't, maybe the idea is to get you to become an active user as the new features are touted as rather intuitive.
Basically, Google+ users could easily build and expand their Circles, while at the same time, filtering emails according to these groups.
First, when you open an email from another Google+ user, you can see the most recent post they’ve shared with you on the right-hand side of the conversation. If they're not already in one of your defined Circles, you can do so directly within Gmail.
Furthermore, when searching through emails, you can see messages sent from people in all of your Circles or individual Circles.
One nifty trick is that if you want to share an image sent to you in an email with another Google+ contact, instead of sending the file as an attachment, the graphic will be uploaded to your Google+ photos stream. It will only be viewable to the Circles that you identify for sharing.
All of the information is supposed to update automatically, thus providing a (hopefully) seamless user experience.
These updates are promised to roll out within the next few days. However, Google Apps users will have to wait while Google continues to work on these updates for those customers.