Tagging is very simple here(google plus tagging)
You can simply tag your friends in any post by putting either the + or the @ symbol in front of their name. Just like Facebook a drop down list will show up once you start typing with suggestions from your friends.
Also, You can tag people you are not connected to which brings me to how to NOT allow people to tag you. This is controlled in your settings panel under “People whose tags of you are automatically approved to link to your Profile:”