A lot of people complained that you can't find the messages sent or received by the members of a group. This feature is still not available, but you can restrict the messages to a circle. Gmail now displays the list of circles below the labels and you can use it to find mail from your family, friends, colleagues etc. It's obvious that groups are now less important than circles, even though I don't think they'll disappear.
"If you've spent time building your Google+ circles, you can now quickly use them to filter your mail, saving yourself from having to sift through that pile of daily deal emails and newsletters. You can see messages from all of your circles at once or from each individual circle. And if you want, you can show circle names on emails in your inbox. Contacts can also be filtered by circles, making it easier to view your social connections."
You can now add someone to a circle from the widget displayed in the right sidebar. There's also an easy way to share pictures from an email to Google+: just click "share" next to the attachment.
{ Thanks, Petros and Joost. }