Yes you can easily tag your post to your friends/groups by just entering their name into the To option available just below your post while posting.
Google Plus automatically detects faces in photos. Mouseover the face for a "Click face to tag" bubble.
Yes it is possible Follow the following Steps 1. First upload a pic 2. Share it 3. Click on the image, a full view of the image will open 4. On top of image you can see a menu in which the third option is "Tag People" 5. Click on it and start tagging Happy Tagging
Like Facebook, Google+ also allows to tag photos with friends. It's easily, you can tag from your Google circles.
No, I don't think its one its possible for now, but who know in the future. Google+ can be line up with facebook now.
Here's 10 steps to tagging people on Google Plus: http://www.wikihow.com/Tag-People-in-Photos-on-Google+
Yes some of social networking sites where can possible tag option in photo of your friends you can get likes in photo